My favorite developers would have to be:
BioWare (Seriously, they've yet to make a game that didn't rock in some form or fashion. No, KOTOR II doesn't count, because they didn't develop it.)
Blizzard (The Two Vikings, Blackthorne (SNES version), Warcraft I, II and III, Diablo 1 and 2, Starcraft 1 and 2, and World of Warcrack. WoW did what no one really thought could happen; they dethroned EQ as the King of MMORPGs)
Valve (Half Life 1 and 2, Counterstrike, Team Fortress 1 and 2, and the very useful tool Steam.)
Bethesda Software (Morrowind rocked, Oblivion sucked, but Fallout 3 and subsequent DLC add ons warshed the bad taste of Oblivion out of my mouth.)