[Aerich]

Well, power comes in different forms. What are we measuring? Sheer ability to deal damage? Skills and class characteristics enabling combat survival? Ability to protect spellcasters? Ability to kill spellcasters?

All that said, I favour the monk. A monk with protection items and appropriate spell buffs has nearly unhittable AC in normal mode, allowing other characters to take up any slack in damage dealing. Combined with the evasion, saving throws, and (eventual) immunity to disease and poison, the high AC makes it the best tank for avoiding damage.

I also like the very high speed and other characteristics that make a monk perfect for taking out enemy casters and archers - stunning blow, rate of unarmed attacks, etc - spell resistance is a great bonus.