Hammerdins, Bone Necros, and Elemental Druids tend to be the most unbalanced characters, IMO. Any jackass can duel and win with them (almost), since 2 of them deal several thousand points of unresistable damage, and Tornado does 4-5k damage/second. Pure cheese.

FoH is resistable, therefore, it isn't totally gay. The best PKer would be a character class that has versatility. I'm too lazy to compare and state my opinion, however.