CCP Games ... but seriously. These guys are superb when it comes to customer support, unlike the other MMO companies on the market. Also, there is that "tiny" advantage over other MMO companies: two (yes... 2) FREE expansions every year.
blizzard- best in terms of providing quality games, games list might be short but once they make a game, it lasts for 10+ years, eg. diablo2, created way back and ppl still playing it, warcraft 3 mhm.
ubisoft - runner up i guess, had some mean games on its list.
Sony, which includes all of their first part developers, from Polyphony Digital (GT), Guerrilla Games(Killzone), Media Molecule(LBP), SCE San Diego Studio (MLB the show), Naughty Dog (uncharted 2), Santa Monica (GOW), to Team ICO (SotC), etc
MicroProse (for Master of Orion)
Blizzard (for Diablo 2 and hopefully 3)
Square (for the old Final Fantasy games)
Nintendo pre-2000, when they actually made good games
Cyan Worlds (Myst)
Bethesda (for Morrowind)
Sonic Team (NiGHTS into Dreams..., Sonic the Hedgehog, etc)
id Software (Open source game engines; doom, quake3, etc)
Sonic Team Jr. (Sonic Robo Blast 2, engine based on id Software's doom engine)
Nintendo (Legend of Zelda, Mario Series)
Konami (Teenage Mutant Ninja Turtles)