Making Halsin a companion really did him a disservice.
I like Wyll when I don't have to talk to him. His biggest flaw however is bringing a devil pest into our camp.
My answer currently is Shadowheart, not because I actually dislike her as a character, but because she's a writer's pet character whose story was used to trash one of my favorites from BG2.
Shadowheart is my second favorite (shares the spot with lae'zel really) and i haven't played BG2, but I also dislike how the game insists on shoving her down my throat, and there's only so much Shar commentary I can handle.