Fighter
None. The Fighter has no special tools to leverage and is slightly disadvantaged against everything
Berserker
Strong: Sorceror/Dragon
Weak: Goo Blob
Warlord
Strong: Goblin
Weak: Meatman/Wraith
Priest
Strong: Wraith/Zombie
Weak: Nothing really. The Priest is flexible and is about average against everything except for undead
Monk:
Strong: Meatman
Weak: Weakening/Corrosion/Resist Down hurts everyone, but hurts the Monk even more
Paladin
Strong: Serpents
Weak: Wraith
Thief
None. The Thief's flexible powers can't be countered by any one ability. The Thief is moderately advantaged against everything
Rogue
Strong: Meatman
Weak: Goblin
Assassin
Strong: Meatman. Also, Swift Hands is the only satisfactory response to a high-level Animated Armor
Weak: Wraith/Zombie Zombie is hard for everyone, but the loss of poison is devastating for the Assassin
Wizard
Strong: Goo Blob
Weak: Meatman
The Wraith can be either strong or weak depending on how much mana the Wizard has and whether or not killing the Wraith will result in a level up.
Sorceror
Strong: Goo Blob
Weak: Wraith, unless mana is very high
Bloodmage
The Bloodmage's approach to resource management is so different that I don't have a good answer to this one
EDIT: Forgot the Church classes
