GTA or MGS4. My guess is, if everything goes like it looks, that by the end of the year the PS3 will have a huge head of steam, while the 360 will start to fall off a lot.
While Gears2 is a strong contender, people don't often give GoTY to dying consoles. (why God of War 2 had no shot of winning last year)
So, if the 360 is still going strong by Jan 09, then I say Gears2. If not, GTA4 or MGS4.
My guess, GTA4







