I'm leaning towards saying Fallout 3. It's the only new property this year that really sucked me in with a vengeance. Even IF the ending was terrible, and makes the game almost unplayable, that "almost" goes pretty far.
Of course, I haven't played Call of Duty 5 or Dead Space.
Wait, wasn't MGS4 released this year? One would think that would be a GOTY nominee.