It is really hard to choose one game out of all the great games already out this year and the ones appearing later. Although I haven't played some of these games, it seems pretty clear judjing from previews and impressions that they are great games. These games are: Fallout 3 - I haven't even played the previous games and I want this; the depth of exploration not seen since oblivion, the great array of quests, the graphics; it just seems brilliant.
Fable 2 - Again, I haven't played the first but this seems like a new step in gaming, with the seamless co - op options, it is sure to be a top contender for GOTY.
GTA 4 - with the move away from the "fun" elements of san andreas *jetpacks* GTA 4 seems to immerse you in its world with a compelling storyline, improved controls and more.
MGS4 - I don't own a PS3 and cant tell much of this, but judging from reviews, it surely must be a contender
Far cry 2 - The exploration seems immense; 50 km of world has me drooling and it seems like a game which will last for ages, also with the replayability value of the map editor.
I probably havent mentioned all contenders - fill in the gaps and tell me what you think!