I'm gonna say Blizzard, other than some ill advised console ports I have yet to play a Blizzard game that wasn't completely top notch.
Valve is a close second but games like Counterstrike and Left 4 Dead just haven't held any interest for me.
Bioware is always hit or miss for me. For every Knights of the Old Republic or Mass Effect there seems to be a Sonic Chronicles or Jade Empire just waiting in the wings to knock them down a few pegs.
Bethesda is always confusing, they obviously put a lot of time and effort into their games but I don't think I've ever played one of their titles where I came away from it happy that I spent the time or money that I did.