Well, obvious answer is Valve--there's a reason their games are universally acclaimed and everybody loves them. However, that's been said many, many times (as has Rockstar), so...
I submit DICE. Sure, the Battlefield franchise has had it's fair share of sequels, but they've added some huge new features along the way (most obviously the destruction mechanics which have been in placed since Bad Company). On top of this, the mechanics are tuned to emphasize teamwork, something which I believe is sorely lacking in most modern shooters. In tandem with the Battlefield franchise, DICE tried Mirror's Edge--and while it may not have been perfect, it was an interesting and innovative stab at something nobody else had really tried (to the best of my knowledge). Almost equally as important, there's a good amount of interaction between the studio and their fans--the Battlefield blog has a pretty steady stream of news, including stuff from the community. Contrast that to the Infinity Ward site which hasn't had a news update in over a year, even with the impending release of MW3.
So... yeah. DICE. Good work, people.