I voted druid, mostly because I've never played one. However, it would depend on the players I was playing with and what classes they were playing.
I could see myself playing a sorceror so the wizard could focus on some more esoteric spells, a fighter with a totally different fighting style from the existing fighter, a paladin of the cleric's god, or a cleric just because I love playing clerics.
The best choice from a metagaming standpoint would probably be Bard but I'm not a big fan of playing the class.
Actually, I think I'd play a spirit shaman from Complete Divine because the class looks really cool from a roleplaying standpoint.