Mine would have to be a toss up between Valve or Blizzard.
They both make fantastic games, i'm a diehard fan of the Starcraft and original Warcraft series(not so much warcraft 3 however) and Valve have given us great titles like Team Fortress 2 and the amazing Steam delivery system which has allowed the independent gaming community to get a hell of alot more exposure and lets not forget the convience of purchasing/downloading directly via Steam.