Definitely GTA4, which doesn't deserve it. Although I haven't played Nintendo games (not ever; not my fault), I have a feeling that even SSBB would be more deserving of the honor, and I'm not interested in SSBB a bit. I like GTA4, don't get me wrong, but it's not even close to a 'revolutionary', perfect game. The camera is terrible and there's really not that much new. Also, the graphics are mediocre at best (well, the style is pretty good but nothing superb there either). Still, a good game but nearly perfect.
If you ask me, MGS4 should probably be the Game of the Year. GTA4 and SSBB are both a bit casual compared to MGS4, which isn't even very hardcore. And if some GTA can get good points and no critic about it's awful camera terrorizing the gameplay, then MGS4 should have nothing to worry about now that the control and camera system has been revamped. But, then again, GTA4 will win most GOTY awards this year, no matter what.