So Dwarf Fortress has seriously changed around the class balance, so much that I'm a bit unsure where everything fits into place. I'm going to make my best guess below, but I'd love to see some other people's thoughts.
High
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Paladin (Still the most solid reliable class)
Assasin (Fireball changes synergize very well with Poison + corrosion boost)
Rogue (Requires JJ due to Dwarf nerf, but very strong with Namtar's Ward)
Warlord (Still solid and reliable)
Wizard (Great with Wharrgarbl, HALPMEH, or PISORF)
Monk (Abundance of magic damage monsters has dropped him quite a bit)
Mid
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Sorcerer (A beast with Fireball combined with Wharrgarbl, Crystal Ball, and/or Fireheart)
Berserker (Unable to efficiently use Fireball, but new magic damage monsters balances that out)
Gorgon (Fireball + Wharrgarbl synergizes VERY well with her built in poison strike - damage is still low though)
Thief (Trisword nerf hurt a lot)
Priest (Trisword nerf hurt a lot, but health buff bumps him up to roughly equivalent with Thief)
Half-Dragon (Capable of a lot of burst damage, but has problems with magic resist monsters)
Fighter (Levels up really, really fast!)
Bloodmage (Provides nothing another class can't get from just finding BLUDPOWA)
Low
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Tinkerer (Provides nothing another class doesn't provide, especially due to item costs being reduced)
Crusader (Probably has some broken Mana Leak combo, but damage boost is too conditional)
Transmuter (Has potential, but balanced far too weak)
Vampire (A Dwarf Crusader who can't get resists. I'm serious - do the math!)