Here are my contenders so far:
Mark of the Ninja: great stealth game whose only real flaw is that a lot of buttons do more than one thing (like how B picks up a body AND lets you hide, leading to situations like a guard seeing you dragging a body and attacks you).
Dishonoured: also a great game, love the emphasis on using magic. However, the game IS very short. I complete a run in which I killed nobody, was never detected AND got at least 75 percent of runes and bone charms in under 10 hours. (Yes, Mark of the Ninja is ALSO that short, but it's also cheaper and made by 11 people rather than hundreds. Also, Mark of the Ninja has more replay value thanks to the different suits)
XCOM: Enemy Unknown: while I haven't played any of the older XCOM games, I had a BLAST with this. While some people find chance-based game-play stupid, I think it only serves to make the game more tense, though it does lead to situations where all your men die in one turn due to unfortunate events involving exploding cars.
Torchlight II: while Torchlight I was good, Torchlight II was AMAZING. It is the best kind of sequel, in that it used the original as a jumping off point. I don't really have a major complaint with this one, just a few minor ones here and there. All in all, great game.
I think I'm leaning more towards XCOM, only because it was the only game that really made me lose track of time. While Dishonoured is amazing, I couldn't play more than three hours at a time without deciding to take a break (this is because of the fact that you had to return to the Pub after beating a mission, which stupidly broke up the action).
I should mention that I haven't gotten around to playing Spec Ops: The Line or Borderlands 2 yet, but from what I hear, they will probably also end up on my list of contenders once I play them.