OK, I will write down what I think about each class and what I think about multi classing them.
I am not an expert builder like Boeroer, but I have collected tons of RPGs.
- Corpse Eater - Targets unconscious enemies to devour their flesh and gain power. Powers cost more to use. ( sorry, I do not understand this)
- Berserker - Has a more powerful Frenzy, but attacks can damage friends as well as foes while Frenzied. (Will be the best chioce if you want to play solo with a barbarien, I would not take it in a party)
- Mage Slayer - Gains spell resistance and can disrupt enemy spells, but cannot use potions or scrolls and beneficial spells have shorter durations. (Too many downsides. A normal barbarian who is good in interrupting should be enough to annoy casters (and all other enemies). I guess there is no creature that is immun to interrupt)
Regarding multi classing, it depends on what works together with carnage and what not.
- Beckoner - Summoning invocations are cheaper and summon more creatures, but the creatures are weaker. (No comment, I have to see how many summons we have and how strong they are.)
- Skald - Offensive invocations are cheaper and melee crits grant phrases, but all other invocations are more expensive. (Seems great for multi classing with high acc melee chars. Like a chanter/rogue with dragon trashed on who uses those cone attacks to knockdown enemies in front of him for easier death attacks)
- Troubadour - Phrase linger is 50% longer, Brisk Recitation as a modal that increases the rate of phrase elapses, but shortens linger. All invocations are more expensive. (For players who like a more passive char. Maybe good in combination with a class that uses mostly auto attacks, like a barbarian that does lots of active and passive AoE)
As single class char, I would use a default chanter, I guess.
- Ascendant - Powers and Soul Whip are more effective when used at Max Focus, but Focus drains quickly if left at Max for long. (Not a good choice for me. You have to attack all the time to reach max focus and you should not cast 2 spells in a row. )
- Beguiler - Illusion powers are more powerful, but Soul Whip suffers when used against targets that are not vulnerable to Sneak Attack. (This can be good. You want to debuff enemies in any case, so the sneak attack condition will be there most of the time. I think this is the best choice for a single class and it may be good for multi classing too. Might be less good when soloing than with a party because more chars make it easier to produce debuffs.)
- Soul Blade - Offensive cipher that can dump Focus into a Soul Annihilation melee attack for extra Raw damage. Shred powers have reduced Focus cost. Lower Max Focus. (I used ciphers mostly as ranged casters. But this can be good in multi classing with a melee char. It might look good in combination with a rogue. First you debuff them, then you use your remaining focus for the finishing blow.)
- Fury - Shift into storm blights and gain bonuses with elemental spells. Cannot cast Restoration spells. (I think this can be good if you want to be a nuker and you also have a healer in your party.)
- Lifegiver - Rejuvenation spells are cast with increased Power Level but cannot cast Summon spells. (Not my first choice. Summon elemental could be very helpful. If you have a party there are also other chars who can help you healing and solo you definitely want summons.)
- Shifter - Druid can shift to any animal form, once each, per combat and heals damage each time they shift back. Cannot cast spells while shifted. (Now this looks good. I mean, you can still cast long lasting spells before shifting. May be good in combination with monk (if mid level claws have higher damage than high level fists) or fighter (weapon specialisation unarmed)
- Black Jacket - Bonus weapon proficiency, reduced Recovery when switching weapon, but lacks Constant Recovery. (Not my best choice. Usually you do not use many different weapon types and the amount of weapon profiencies of a normal fighter should be enough.)
- Devoted - May only be proficient in a single weapon. Higher Penetration and crit damage with that weapon. Suffers Accuracy penalty when using other weapons. (Initially it may hinder you too much. At the start you have to use whatever weapon you may find. But if you find a very good weapon later this can be very good. Like a ranger/fighter once you find a ranged weapon with 2 damage types.)
- Unbroken - Bonuses to Engagement and Disengagement Attacks, but lower Stride. (Low movement speed seems to be a huge penalty. In combination with monk (movement abilities) or paladin this may become the ultimate tank.)
- Helwalker - Begin all combats with Wounds, gain Might for every Wound. Wounds require more damage to acquire. (I disliked that you have to get hit to use abilities and I hate to get increased damage even more. I do not think I will ever use this.)
- Nalpazca - Drug effects last longer and Wound cap is increased while under the effects of drugs. Penalties while not under the effect of drugs. (No comment, I did never use drugs.)
- Shattered Pillar - Gain Wounds by inflicting damage with melee weapons (fists or otherwise). Lower Max Wound cap, does not gain Wounds from receiving damage. (When using a monk, this might become my first choice. This looks very good if you combine the monksterlasher build with a barbarian. With carnage you create many hits, creating wounds fast. Thanks to the wounds enemies will take lost of additional elemental damage. Thanks to AoE interrupt you will not get hit a lot, so you would not gain many wounds by taking damage anyway. Add barbaric retaliation and heart of fury to the mix.)
- Bleak Walkers - Flames of Devotion (Remember Rakhan Field) generates black flames and does Corrode damage. Healing given and received is reduced. - Cannot multiclass with priests of Eothas, Berath, or Skaen. (Less healing sounds bad. Not the best choice for me.)
- Darcozzi Paladini - Lay on Hands (Flames of Darcozzi Palace) creates a flame shield around the paladin. Lower Zeal power. - Cannot multiclass with priests of Berath, Magran, or Skaen. (No comment, never used them in PoE)
- Goldpact Knights - Sworn Enemy (Gilded Enmity) creates protective gold armor on the paladin. Cannot learn Zealous Auras. - Cannot multiclass with priests of Magran or Wael. (Nothing fo me. Passive auras are a fundamental part of paladins, at least for me.)
- Kind Wayfarers - Flames of Devotion (Sword and the Shepherd) heals nearby allies. Does less damage against enemies vulnerable to Sneak Attack. - Cannot multiclass with priests of Berath, Magran, Wael, or Skaen. (This is bad. You want to debuff enemies in any case. If this char is weaker against enemies who met the conditions that are required for sneak attacks, he will be weak against most enemies most of the time. This is very sad because I wanted to play a kind wayfarer in PoE1 and continue with him in PoE2. He might change to be a shieldbearer in the second game.)
- Shieldbearers of St. Elcga - Lay on Hands (St. Elcga's Grace) prevents the target from being knocked out for a short duration. Cannot use Lay on Hands on self. - Cannot multiclass with priests of Skaen, Magran, or Wael. (Since you cannot have a paladin without order, this seems to be best choice for me. Just to be sure, "knocked out" means zero endurance, not prone.)
- Berath - Can learn Decay spells from the druid list, cannot learn Condemnation spells. - Cannot multiclass with Bleak Walker, Kind Wayfarer, or Darcozzi paladins.
- Eothas - Can learn Elemental spells from the druid list, cannot learn Protection spells. - Cannot multiclass with Bleak Walker paladins.
- Magran - Can learn Fire spells from the wizard list, cannot learn Restoration spells. - Cannot multiclass with Shieldbearer, Kind Wayfarer, Goldpact, or Darcozzi paladins.
- Skaen - Can learn rogue offensive abilities as priest spells, cannot learn Inspiration spells. - Cannot multiclass with Shieldbearer, Bleak Walker, Kind Wayfarer, or Darcozzi paladins.
- Wael - Can learn Illusion spells from the wizard list, cannot learn Punishment spells. - Cannot multiclass with Shieldbeaer, Kind Wayfarer, or Goldpact paladins.
- Ghost Heart - Animal companion must be summoned as a spirit. They are not affected by Bonded Grief and the companion is more powerful, but the summon is limited duration. (I like this. The usefulnes depends on the lengh of the summoning vs the lengh of the battle.)
- Sharpshooter - Bonuses to Penetration and Accuracy at range, but slower actions and lower Deflection. (Looks interesting, especially if you combine it with a devoted fighter and a wood elf.)
- Stalker - Stalker and companion gain bonuses to Deflection and Armor Rating when close to each other, suffer Bonded Grief when too far apart. (May be interesting for some players, but I will not take it.)
I like a combination of ranger and rogue. Pet causes flanked for sneak attack, deep wounds causes dot so the pet does more damage. Even more useful if death attack works together with twinned arrow.
Otherwise I would prefer single class rangers, so you have a very powerful pet that acts as a permanent full party member.
- Assassin - From stealth or invisibility, weapon attacks have bonus damage, Penetration, and Accuracy. All incoming damage is increased. (On its own it feels a little too fragile. May be very interesting in combination with protection spells like a priest of wael or an illusionist wizard. Especially if death attacks will still work with spells (but I consider this to be a bug)).
- Streetfighter - Sneak Attack and crit damage increases when Flanked or Bloodied (<50% Health). Recovery is slower when neither Flanked nor Bloodied. (Sorry, I dislike to be flanked and I also dislike chars whos special things work only if they have und X health. I prefer to be at full health all the time. Some people may like this, but not me.)
- Trickster - Gains access to Illusion spells from the wizard list. Sneak Attack deals less damage. (Less sneak attack damage is bad for a rogue. I would prefer a rogue/Illusionist over a rogue with lower damage. I would even prefer a default rogue over this, because you can get most effects via scrolls, potions or another char in your party.
- Conjurer - Conjuration spells are more powerful, cannot cast Evocation or Illusion spells. (May be useful in combination with a devoted fighter. Having an empowered citadels lance may be nice.)
- Enchanter - Enchantment spells are more powerful, cannot cast Illusion or Transmutation spells.
- Evoker - Evocation spells are more powerful, cannot cast Transmutation or Conjuration spells.
- Illusionist - Illusion spells are more powerful, cannot cast Conjuration or Enchanting spells. (May be useful in combination with melee chars who want more protection.)
- Transmuter - Transmutation spells are more powerful, cannot cast Enchantment or Evocation spells.