I think Butters has a good mix of attacking ability and utility, since he can heal the player... but he can't use the healing power on himself, so that's a drawback. I like Stan too.
I guess something to keep in mind is that the buddies comment on things in the environment... I like hearing Butters' comments, he has some great ones. Kenny's mumbling is funny, but at the same time, almost totally unintelligible...