My personal favorites are Id Software, 3DO ( rip =( ), Wherever the lisence to produce D&D based games goes- ( Atari, Bioware, Interplay, Black Isle Studios...... god knows what the hell else yet they are always fantastic games. ).
Also Capcom and Squaresoft, NOT Square-enix.
And I guess Blizzard even though after they release a great game they chronically make me angry with thier future decisions.
I hate Blizzard, but they put out better games than I've seen from any other company. Unlike most other companies, Blizzard never releases a crappy game - and they continue support for their games pretty much infinitely. They know how to keep a game alive and fun for years and years, something that the world of episodic gaming seems to have trampled upon.