Right now, EA is the best, because Bioware is on top of it's game, EA has also been branching out and taking risks with unestablished games like Brutal Legend and Mirror's Edge.
Worst? Activison seems to have only the almighty dollar in mind, and players are starting to realize just what that means. COD every year? Guitar Heros every other month? No thanks you soulless corporation.
EDIT: Note I said Activision, not Activision-Blizzard. I think the guys over on the Blizzard side seem to know what they're doing (except the RealID thing), and are giving their fans what they want.