so far, what we know of...

Wii: Brawl

PS3: MGS4

360: Gears 2

it could change in the upcoming months with more games coming

The top contender's I can see possibly winning GOTY for Gamespot are:
1.MGS4
2.GTAIV
3.LBP
4.SSBB (maybe, I doubt it)

The only other contender's that have a shot at it are The Last Remnant and Fable 2 (these two at least have a shot at RPG of the year).

GTAIV is looking like a contender however I think it will have a Halo 3-esque reception.
Brawl probably won't be a contender due to its style and the fact that it wasn't actually perfectly pulled off.
MGS4 won't be a contender as its too heavy and too niche, a stealth game with a deep plot just won't get it in my opinion. Gears 2 might be a contender but I just don't think its going to live up to its predecessor.
Resistance 2 probably won't be a contender due to the fact that Resistance 1 wasn't actually a blow your socks off good game.
Little Big Planet I just don't feel is big enough, may have a shot however if its pulled off really well.
WiiFit I feel has the best chance of currently known games but it really depends on how western reviewers accept the idea of a game based on fitness.

DS FF4
PC Spore
PS2 GH4
PS3 MGS4
PSP FF7 or GOW
Xbox 360 Fable 2
Wii SSBB

or GTA4 could always win

well game of the year awards are kinda random....last year i honestly thought call of duty 4 should have gotten it rather than bioshock on most sites but thatz just my opinion... but this year probably GTA IV or maybe an unknown title not announced yet...

How can Guitar Hero define this generation if it's been coming out for PS2 since before the 360 came out?

On topic, it sounds like MGS4 to me as of right now. 

W/O Bias: Wait more time. After all, we have not fully experienced the games yet except for Brawl. We have to wait until the games are out, have been played, has been beaten, replayed, beaten again, and replayed again to exploit its mechanics. THEN we can decide.

W/ Bias: BRAWL FTW!

I think either:
-Gears of War 2
-Metal Gear Solid 4
-Grand Theft Auto 4

Right now it'll be between SSBB, MGS4, and Gears 2. I really think people are just having wishful thinking when saying Killzone 2 and Resistance 2 will be AAA titles.

Wii Fit without a doubt.

of course if your referring to what games will be awarded that title by gaming magazines and websites? then the answer is who cares! If they're not going to recognize that Wiifit is the game of the year they can go stick it up their left nostril!