Nintendo is totally my favourite. Mostly because of Pokemon and Zelda. Also, the games are just generally good, even Avatar: The Last Airbender, which controlled like butter and had an awkward battle system.
A couple of years ago I would have said Capcom(Street Fighter, Resident Evil, Mega Man) but seeing the crap they have pulled for the last few years that one is out, same with Konami(Castlevania, Silent Hill) although they are not as bad.
Nintendo and all of their studios are way high
Valve is also very good
Bathesda and Bioware(mostly 2010 and back)
Square Enix, of course