I'd have to say Bungie. Other devs have made games that I enjoyed more, but as far as the actual people that make up the studio, they top my list.
I loved Marathon and Oni, and kept on liking them with Halo.
They seem to do a good job of not taking themselves too seriously, laughing at their mistakes, and their community team does a fantastic job. I listen to podcasts -which I find phenomenally funny- and extra material (the remastered H1 and H2 cinematics were awesome) whenever I can, and Joe Staten and Marty O'Donnell seem like pretty cool guys. Who kill aliens and blah blah blah...