Definitely Nintendo for me, by far. They've already confirmed playable Cafe at the show, so it will undoubtedly have a huge presence at their conference. What I'm really hoping for is a lot of surprise third-party announcements pertaining to Cafe. And (hopefully) some really great stuff on 3DS.
Sony will almost certainly be second, given the NGP and ND showing off their "blow you away" gameplay demo of Uncharted 3 like they did with U2 back in '09.
Microsoft may try and steal some of Nintendo's thunder with some kind of announcement regarding Xbox 3, but no way will they have anything playable, so I don't expect them to shine until next year. Personally I don't really care about Kinect either, so unless they have some huge surprises they'll likely be a distant third for me.
