1) Portal 2
2) Deus Ex: Human Revolution
3) Dragon Age 2
4) Batman: Arkham City
5) Alice: Madness Returns
I'm not including games I haven't played yet. Why would I? I expect to love Skyrim, Assassin's Creed: Revelations, and especially The Legend of Zelda: Skyward Sword. But I don't know that for sure yet.
And if I could count New Vegas' Old World Blues as a separate game, it would be #1 on my list.