Ninpanda on 14 May 2011
Polyphony Digital for Sony, without a shadow of a doubt. I might reconsider once a Sony home console comes along, whose most successful first-party title isn't a Gran Turismo game.
For Nintendo, I'm not sure. Nintendo EAD Group 2, 4, and 5 (The creators of Wii Sports, the NSMB games, and Wii fit, respectively) are all invaluable.
Don't know much about Microsoft's first-party studios.
