I was just watching some development videos on Starhawk and I was surprised to see that Sony Santa Monica's logo was shown at the begining of the clip and it went on to show that they had a big hand in helping LightBox Interactive with Starhawk. That got me thinking, Santa Monica has been Sony's go to studio for all their development needs, they helped out with Little Big Planet 2, they collorborated on Warhawk and now Starhawk, they had a hand in all the God of War PSP games (and the mobile game), Sony send them out to help other developers with bringing their games on to the PS3 and they also develop the main line God of War games. They seem to be the most useful studio in SCE's line-up.
Others may argue that Naughty Dog have created more IP's and have currently created a bigger franchise for Sony, that Japan Studio release much more content and run PSN or that Cambridge is their go to team for things like research, non-game software development and Home.
I personally think that Santa Monica is their most useful studio. Second Party studios can be included in the discussion.
I don't know enough about Microsoft and Nintendo, but I'd guess that Rare and EAD Group 5 (boring name) are their most useful studios.
Bet with Conegamer and AussieGecko that the PS3 will have more exclusives in 2011 than the Wii or 360... or something.