All right, you can mention if HMX really, genuinely does top the list for you, but I think it's sensible to set them aside because they are much more specialized than other developers.
I will set HMX aside (because I'm not sure how I'd rank them), and say my favorite developer is Ubisoft. I have yet to play an Ubisoft game that I didn't love. Of course Assassin's Creed tops the list, but also Beyond Good & Evil, Prince of Persia, the new downloadable game I Am Alive, I even played a couple of the Tom Clancy games on the previous generation's consoles. A long winning streak with me and Ubisoft.
Team Ico also bears mention, without question. What they lack in quantity they certainly make up for in quality (Ico and Shadow of the Colossus being their only existent games so far) and I am greatly anticipating The Last Guardian.
Honorable mention to Crystal Dynamics for Tomb Raider. It's been a while since they developed anything else.
Bioware used to be high up on this list, but.....you know