My vote here likely isn't the most valid, as I have yet to experience many of the games that have released this year. However, that aside, I have to go with God of War III.
It isn't because of the action, or the graphics, though. It is the implementation of a solid story, that works in every way, no matter how you look at it. Many games fail to implement a story that works forward and backwards, meaning there are various flaws here and there that show inconsistency in the story.
If I had to take a second vote, however, I'd like vote for Mass Effect 2, for obvious reasons. The implementation of various environments, realistic characters and dialogue, it really makes you feel like you're a commander traveling the universe from what I've seen of it so far.