As I've learned through repeated playthroughs, some masteries are stronger than others and better for levelling up in the game, with certain skills to use in order to use to gain enough experience to learn new skills. Some masteries, on the other hand, are not so good for this purpose.

I'll list all eleven masteries in what is my opinion from strongest to weakest.

1. Nature - The wolves summoned by Call of the Wild practically do all the work for you. They can be further strengthened by Survival Instinct and Strength of the Pack. Earthbind can reduce enemy defensive ability allowing them to score more critical hits. Susceptibility can reduce enemy resistance for more damage. Heart of the Oak can increase health and speed for yourself and your minions. The wolves get even more effective if you can get three of them. They do start to suffer against certain bosses later on in the game, so you may need skills from another tree to provide further buffs, debuffs, or give yourself another damage skill.

2. Storm - Squall is one of the best crowd control skills in the game, but it needs to be maxed for maximum radius. You can strengthen it further with Eye of the Storm and Static Charge to increase the lightning damage. Obscured Visibility can reduce enemy resistance.

3. Rune - Runic Mines are good for offense and defense, just cast them and lure enemies onto them. Rune Weapon can increase all elemental damage and you can increase your damage with weapons with Magical Charge. Later, you can pick up Seal of Fate for high elemental damage, though you'll need Menhir Wall to pair off with to block enemies and prevent them from fleeing. Aftershock can reduce resistance and inflict other debuffs. Transmutation adds more damage by converting weapon damage to elemental.

4. Dream - Distortion Wave is great for slowing enemies and dealing damage, making it a great crowd control skill. Chaotic Resonance and Psionic Immolation both add more damage and reduce recharge. Lucid Dream adds more damage as well. However, this combo costs a lot of energy, so be sure to get equipment with energy leech. Psionic Touch can be good for a left-click attack. Trance of Convalescence can also be good for survivability and rejuvenation.

5. Neidan - Dragon's Breath is the best skill to invest in at the start. Later on, you can respec the points into Dragon's Flame. Shen Pao is another great damage skill for more damage and has a low recharge value, though it needs to be maxed for the highest radius of effect. Weakest Chakra, Consequences, and Mutual Suffering all reduce resistance. Huangdi's Favor is good for a left-click attack. Inner Equilibrium increases speed as long as you aren't under the effect of a potion.

6. Earth - Ring of Flames is the best choice for levelling up, though it needs to be maxed for the highest radius. Maxing Earth Enchantment and Volativity is good for increasing the damage. A point in Stone Form and Heat Shield is good for increased survivability and defense. However, Ring of Flame starts to become less effective late Act 3.

7. Warfare - Of the masteries suited primarily for combat with weapons, this one is the strongest. Battle Standard gives damage buff and Triumph reduces resistance. Weapon Training increases weapon attack speed and offensive ability. Battle Rage and Crushing Blow provide temporary damage buffs. Onslaught, Hamstring, and Ardor is great for a left-click attack. A point in each skill in the Dual Wield tree increases effectiveness in fighting with two weapons. A point in War Horn is good for utility.

8. Rogue - Poison Gas Bomb with Shrapnel is great for crowd control, though PGB needs to be maxed for highest radius. Nightshade and Toxin Distillation increases poison damage. Calculated Strike and Lethal Hit are great for dealing with enemies immune to poison and bleeding damage, including the undead, constructs, devices, and traps. Disarm Traps is good for increasing damage against constructs, devices, and traps. Anatomy increases bleeding damage inflicted by Lucky Hit and Shrapnel. Casters who can get four extra skill levels from equipment should consider picking up Throwing Knife and Flurry.

9. Defense - Batter is the best skill to level up at the start, but it becomes better with Rend Armor. Shield Charge's Disruption needs to be maxed for attacking multiple enemies around you. Colossus Form and Defensive Reaction both increase physical damage. The passive shield attacks are all useful, especially Pulverize.

10. Hunting - Marksmanship is the best attack skill, but it can only be used by bows or throwing weapons. The synergies are both very good. Study Prey reduces physical and piercing resistances. Wood Lore increases defensive ability and attack speed with bows, spears, and throwing weapons. Volley is a powerful triple attack. Spear users can later pick up Spear Dance and Take Down. Flush Out reduces bleeding resistance and defensive ability. Ensnare with Barbed Hooking can be good for crowd control. Ensnare needs to be maxed for increased area of effect.

11. Spirit - Liche King and its synergies are the only choice for helping you level up because the undead, traps, constructs, and devices are all immune to vitality damage. Since the Liche King is hampered in battle against the undead for this reason, Spirit Ward and Spirit Bane are both necessary. Unearthly Power can increase the damage inflicted by you and the Liche King. Later on you can pick up the Outsider. Circle of Power reduces energy cost, recharge, while increasing casting speed, and both vitality and bleeding damage. Once you get more powerful staves, you can pick up both Ternion Attack and Arcane Lore. The Deathchill Aura and its synergies increase survivability because Ternion Attack with Arcane Lore is best used at close range. Life Drain is more useful for healing than actual damage.

Future posts to this thread will reveal what I think is the best mastery to start off with for each class.

Pure Strength: Berserker, Champion, Conqueror, Daoist, Dreamkiller, Guardian, Harbinger, Monk, Spellbinder, Spellbreaker, Templar
Pure Dexterity: Bone Charmer, Brigand, Haruspex, Illusionist, Pilgrim, Ranger
Pure Intelligence: Channeler, Conjurer, Contemplator, Diviner, Druid, Elementalist, Esoterist, Evoker, Hermit, Oracle, Prophet, Ritualist, Runesmith, Seidr Worker, Shaman, Skinchanger, Soothsayer, Sorcerer, Spiritualist, Stonespeaker, Summoner, Thunderer, Warlock, Wu
Strength and Dexterity: Assassin, Corsair, Slayer, Warden
Strength and Intelligence: Battlemage, Juggernaut, Paladin, Thane
Dexterity and Intelligence: Avenger, Disruptor, Dragon Hunter, Sage, Trickster

Elementalist: You get one of the highest base intelligence with this class. Together with dexterity, it is raised by both masteries. Intelligence is the only stat among them that has skills to inflict damage based on it. Funnel those points into intelligence with enough dexterity for equipment. This caster has some of the most powerful fire damage in the game of all classes.

Esoterist: Intelligence and dexterity are both raised by both masteries. Focus on intelligence for damage and dexterity for requirements.

Evoker: Intelligence is raised by both masteries. Raise it primarily for damage and dexterity for meeting requirements.

Guardian: Dexterity is raised by both masteries, but the damage skills raised by it are few. There are no skills to inflict intelligence-based damage despite it being higher than strength, but strength has many options, so raise it. Dexterity is only for meeting equipment requirements.

Harbinger: Strength is the commonly raised stat by both masteries. You have the highest base strength of all classes with this one. Many skills in Dream inflict both physical and elemental/vitality. Raise strength for damage and dexterity for equipment.

Haruspex: Strength is raised by both masteries, but there aren't as many options for inflicting primarily physical damage, but dexterity-based damage is more common. Raise dexterity for damage and strength for equipment requirements. Strength is also good for increasing the damage of physical damage skills.

Hermit: Intelligence and dexterity are both raised, but intelligence has more sources of damage inflicted by it, so funnel those points into intelligence and raise dexterity for equipment requirements.

Illusionist: Both masteries raise dexterity. Rogue is revolves around dexterity-based damage. Since Nature has a debuff for poison damage, this is the best poison damage caster. Dexterity is your primary stat for damage. Strength is better to raise for equipment requirements as there is nothing to benefit intelligence.

Juggernaut: Dexterity is raised by both masteries. Defense focuses more on strength-based damage, while Earth focuses on both strength and intelligence while raising only intelligence. Strength is just as important as intelligence for damage.

Magician: Dexterity is raised by both masteries, but only Rogue is dexterity-based damage, while Earth is intelligence-based. As you can imbue your throwing knives with elemental damage, intelligence is more important for damage with dexterity for meeting requirements.

Monk: Both masteries raise dexterity, while Defense raises strength and Neidan raises intelligence. There are more sources of strength-based damage over intelligence-based damage. Strength is more important for both damage and meeting requirements since shields require strength. Dexterity also needs to be raised to meet equipment requirements.

Oracle: Intelligence and dexterity are both raised, but only intelligence has damage sources, so raise it for damage and dexterity for requirements.

Paladin: Dexterity is increased by both masteries. Defence focuses on strength, while Storm emphasizes on intelligence. Both strength and intelligence are equally important for damage and requirements.

Pilgrim: Dexterity is raised by both masteries. Hunting raises strength while Neidan increases intelligence. Strength, despite being lower than intelligence, is better for raising to meet equipment requirements.

Prophet: Intelligence is the commonly raised stat by both masteries. There are more skills inflicting intelligence-based damage than strength-based. In fact, most of those in Dream inflict both types. You can get the highest electrical burn damage with this class. Funnel points into intelligence for damage and dexterity for requirements.

Ranger: Both masteries increase dexterity. Since there are no skills that inflict intelligence-based damage, aside from Detonate, raise strength for item requirements and dexterity for damage.

Ritualist: Both masteries raise intelligence. Many skills in Dream that inflict physical damage also inflict intelligence-based damage, though the physical damage isn't as high as the electrical burn damage. Raise dexterity for requirements and intelligence for damage.

Runesmith: Both masteries raise strength and dexterity. Rune also increases intelligence and has a higher emphasis on intelligence than strength. Transmutation converts a portion of all physical damage on weapons and shields to elemental, so raise intelligence for damage, with strength and dexterity for requirements.

Each class seems to be best built for one type of damage and style. The key to determine how to best build your character is picking skills that synergize with each other in both masteries. Another way that I will discuss in this thread is which stats to focus on for each class, both primary for increasing your damage, and secondary for wearing your gear.

Using a Titan Quest Calculator, you'll find that certain mastery combinations each have one or two stats that are raised by both masteries. Pure warrior classes that fight with weapons raise both strength and dexterity. Pure caster classes raise both dexterity and intelligence. Hybrid classes that use one fighter mastery and one caster mastery raise all three, but for these combinations, usually, the only common stat raised is dexterity, though many such classes have few or no skills that inflict damage raised by dexterity. Dream mastery is the only one that does not raise dexterity. Rune increases all three stats.

There are six groups that each class can fit into one of. They are:

Pure Strength - Physical damage
Pure Dexterity - Piercing, bleeding, or poison damage
Pure Intelligence - Elemental or vitality damage
Strength and Dexterity - Physical, piercing, bleeding, and poison damage. May or may not use all of them.
Strength and Intelligence - Physical, elemental, and vitality damage
Dexterity and Intelligence - Piercing, bleeding, poison, elemental, and vitality damage. May or may not use all of them.

For example, Dream, being a hybrid mastery has skills that inflict both physical and either elemental or vitality damage. Earth is a caster class that focuses primarily on fire, though many skills also inflict physical damage at the same time.

Some hybrid classes only raise dexterity, but the class might not have many or skills that inflict dexterity-based damage. If that's the case, look at what type of damage is inflicted by other skills. For example, both Nature and Warfare raise dexterity, but there are only two skills that inflict poison damage and the damage is quite minimal. But Warfare does have skills that inflict physical damage. Despite intelligence being raised, there are no skills that inflict elemental or vitality damage.

If both masteries have skills of different damage types raised by two of the stats, it might be beneficial to use both. This is true for hybrids such as Thanes, Paladins, Juggernauts, and Battlemages.

Damage over time can be good for secondary damage, but damage, where the full amount is dealt instantly, is more powerful and overall better.

Future posts on this thread will list the primary damage stat or stats for each class. And the classes will be put into one of the six categories.

Some questions about raising skills or the mastery bar:

-When an important damage skill becomes available (such as Call of the Wild), a skill that improves the damage of said skill (such as Strength of the Pack), or reduces resistances of enemies to that damage (such as Susceptibility), should I max the skill first before continuing to climb up the mastery bar?
-How do I know when to climb up the mastery bar instead of raising skills?
-When should I invest in defensive skills?
-How do I know if a skill should only have one point when it first becomes available?
-If playing a warrior using weapons, when should I get skills that inflict percentage-based damage such as Onslaught, Thunder Strike, Calculated Strike, or Psionic Touch?

Sage - Elemental Bow/Spear

Elemental-imbued bows from Storm Nimbus and its synergies are used from range with Spears and Spear Dance when surrounded. Obscured Visibility and Flush Out reduce resistances. Eye of the Storm increases all elemental damage. Lightning Bolt and Chain Lightning are good for crowds. Pure spear users have no spammable left-click attack. Physical Sages will lack the punch offered by elemental damage. A pure caster would also not be able to make use of most skills in Hunting.

Seidr Worker - Elemental Dualist Throwing Caster

Rune Weapon increases electrical burn attacks, making it the superior choice of attack compared to Psionic Touch and Psionic Beam. Aftershock and Hypnotic Gaze can reduce resistances.

Shaman - Elemental Dualist Throwing Caster

Rune Weapon is superior to Ternion Attack because it increases both elemental and vitality damage. Unearthly Power gives more bonuses to elemental and vitality damage. Aftershock and Necrosis both reduce vitality resistance. Vitality damage is better used to heal yourself and certain enemies including the undead and constructs are immune to it.

Skinchanger - Elemental Dualist Throwing Caster with Pets

Susceptibility can help reduce elemental resistances together with Aftershock and the wolves make great distractions for your spells. Enhance the defenses of your pets with Rune of Life and Energy Armor if you can afford the energy cost with enough equipment to reduce it.

Slayer - Dualist Throwing/Spear

Throwing Weapons gain a bonus to attack speed and both offensive and defensive ability from Wood Lore and Weapon Training, making them superior to bows for this class. Spears can be used with both War Wind and Spear Dance. Battle Rage and Battle Standard increase all physical damage which increases the piercing damage done by your weapons when the buffs are applied. Using dual melee weapons won't allow you to use certain combat skills skills in Hunting.

Soothsayer - Vitality Ternion Caster with Pets

Wolves can help deal with enemies that are immune to vitality damage. Unearthly Power increases their damage and Susceptibility helps reduce resistances that Necrosis can't. It might also be beneficial to use the Eitr charm to add elemental damage to at least one of your jewelry for inflicting elemental damage.

Sorcerer - Elemental Knife Caster

Elemental damage is better for imbuing your throwing knives than poison, even though Obscured Visibility reduces both elemental and poison resistance. You can still apply the status ailments from Mandrake and Nightshade though. Lightning Bolt is good for crowd control and Eye of the Storm increases all elemental damage. The traps can also benefit from the Storm Nimbus tree. A melee sorcerer won't work very well due to Throwing Knife being a safer attack.

Spellbinder - Physical Melee with Shield

Buffs from both masteries increase physical damage, but they can also increase bleeding damage as well, even if there aren't any skills in either mastery that can inflict it. Using a staff with Ternion Attack will not allow you to use any skill that requires a shield.

Spellbreaker - Physical Dualist Melee

Both masteries have skills that increase physical damage, though some skills can deal bleeding damage, even if they are few and the damage minimal. Using a staff with Ternion Attack will not have any benefit from most skills in Warfare.

Spiritualist - Vitality Ternion Caster

Ternion Attack is superior to Huangdi's Favor with Spreading Influence due to the wider area of effect and the short range, allowing enemies to get in range of Necrosis. Weakest Chakra can inflict vitality damage as well as reduce resistances. Consequences can reduce resistances further with weapon attacks. You might want to use an Eitr charm to can add elemental damage to your attacks because some enemies are immune to vitality damage. Using elemental attacks can reduce your damage per second as Ternion Attack is a more powerful and faster attack.

Stonespeaker - Elemental Dualist Throwing Caster

Earth Enchantment, Brimstone, and Volativity increase the fire portion of all elemental attacks in Rune while Rune Weapon can increase all fire damage spells in Earth. Aftershock reduces resistances. A physical caster isn't as strong because Rune Weapon does not increase physical damage. A pure fire caster outside of Rune Weapon will not be as powerful either.

Summoner - Elemental Caster with Pets

Brimstone increases the damage of wolves. Flame Surge can reduce enemy defensive ability as can Earthbind. The Core Dweller is a great addition to your minions. Volcanic Orb and Eruption are excellent crowd control skills. Strength of the Pack increases physical damage of the Core Dweller and physical damage skills. Susceptibility reduces elemental and physical resistances. Since neither mastery increases strength and the total bonus to elemental is higher than physical damage, a physical caster isn't as powerful.

Harbinger - Physical Dualist Melee Caster

Both masteries offer physical damage and increased bonuses to it. Hypnotic Gaze can also reduce physical resistance. All physical damage spells are affected by Lucid Dream, including Slam. Dream Stealer is good for rejuvenation when Phantom Strike is used. Phantom Strike is superior to War Wind in terms of damage and targets. Warfare doesn't offer anything to benefit electrical burn damage. Using Psionic Beam with a staff dealing physical damage doesn't qualify you for certain skills in Warfare. There also aren't very many staves that can inflict physical damage.

Haruspex - Piercing and Physical Bow/Spear

Increased physical damage from Lucid Dream can have an impact on the increased piercing damage inflicted by bows and spears. Phantom Strike can be used with spears together with Spear Dance. Study Prey and Flush out can reduce resistances. Spears can be used with Psionic Touch and Psionic Beam for reduced charge levels, but Marksmanship is better for ranged attack with spears useful when surrounded. Using a staff with Psionic Beam won't get much use out of the majority of Hunting's skills.

Hermit - Huangdi's Elemental Caster with Pets

Terracotta Servants make excellent additions to your minions and they receive a bonus to damage from Strength of the Pack. Susceptibility works together with Weakest Chakra to reduce resistances, as can Consequences when using a staff with Huangdi's Favor and Spreading Influence. Pets cannot inflict Consequences, but enemies killed by them under its influence can trigger Mutual Suffering. Elemental attacks are good for aiding your minions and the spells can also inflict Consequences. Since neither mastery affects strength, a melee Hermit isn't very effective.

Illusionist - Poison Knife Caster with Pets

Earthbind deals poison damage which is increased with Nightshade and Toxin Distillation. Wolves and traps can distract your enemies and inflict damage against enemies immune to poison. Envenomed Throwing Knife with Flurry deals more poison damage thanks to Susceptibility's ability to reduce poison resistance. A melee Illusionist wouldn't work as well as a knife caster is more efficient.

Juggernaut - Hybrid Melee Caster with Shield

Brimstone increases all physical damage of shield attacks, while Defensive Reaction and Colossus Form increase all physical damage spells in Earth. Many spells that inflict fire damage also inflict physical damage, such as Volcanic Orb and Meteor Rain. Earth Enchantment also imbues your weapons and shield attacks with elemental damage. Volativity increases fire, burning, and physical damage. Defense offers nothing for a fire caster.

Magician - Elemental Knife Caster

Imbuing Throwing Knife and Flurry with fire and burning damage and using fire spells at range such as Volcanic Orb make for an effective fire caster. The traps benefit from Earth Enchantment and Brimstone to have more damage inflicted. A melee magician isn't as effective or strong as Throwing Knife is more effective with imbued fire damage.

Monk - Physical Melee with Shield

Nothing in Defense offers Neidan's elemental damage so physical melee is the way to go. The defensive buffs and enemy debuffs work well in melee combat. Consequences can be inflicted with your attacks.

Oracle - Elemental Ternion Caster

Both masteries offer elemental buffs and an ability to reduce vitality resistance. Elemental resistance is also reduced by Obscured Visibility. Ternion Attack with Arcane Lore is better overall than Ice Shard due to the shorter range allowing enemies to get in range of Necrosis. Pure vitality isn't effective either because nothing in Storm benefits vitality damage aside from the reduced resistance, and it's better for healing yourself.