I haven't played all the big ones that came out this year, and at least in the case of Arkham City and Human Revolution, I feel like I'm missing out (I'm considering buying Arkham City when it comes out for PC, though).
As it stands right now, my choice may seem obvious to you, but it's a big surprise for me: Skyrim. I hated Oblivion, and I never had the chance to go deep into Morrowind. I was seriously expecting Skyrim to be more of the same as Oblivion, but for me, at least, it wasn't. I'm currently playing the sh*t out of it, and unable to stop and play anything else. And I still have more than half of New Vegas ahead of me!
Another candidate would definitely be Portal 2. I loved that game to death.