I can't pick one so I'll pick three. Rockstar Games, Square Enix, and Blizzard.
Why? each of them made iconic, legendary, unique games that have heavily influenced the gaming world in one way or another.
Rockstar Games: all the GTA series, Max Payne series, Red Dead Redemption, Bully, ... every single game I played from them I absolutely loved. They're really good at what they do and know how to please the players.
Square Enix: all FF series, Kingdom Hearts series, Just Cause 2, ... I'm a big FF fan so it's impossible not to love Square Enix. They're unique in what they do and present. amazing in-game cinematics.
Blizzard: World of Warcraft, Starcraft, Overwatch, Diablo, ... they're probably the most amazing company when it comes to MMO's. I think everybody played at least one of their game at some point.