X-Com: Enemy Unknown - I still install this at least once every year or so...
Assassin's Creed 2 / Assassin's Creed:Brotherhood - Loved them both, in fact the only things that would pull Brotherhood ahead are minor gameplay changes and the barely-used Multiplayer.
Minecraft - Addicting as hell, period.
Honorable Mention:
Portal / Portal 2 - GLaDOS still tops my list of Best Antagonist in a Game Series.
Uncharted Series - I'm a sucker for a fun story and puzzles, and let's face it, the environments are frikkin' gorgeous.
Batman: Arkham Asylum - Fun to explore, engaging story, and being able to pound in skulls as Batman... what's there to not like?
Metal Gear Solid Series - 3,4,1,2 in that order... MGS1 would probably move up to spot #2, if I could get past the fact that it looks absolutely horrid on the new tv.
Red Dead Redemption - Just me, my steed, and my long rifle... and an open plain. Oh sh*t, cougar. :

anics and shoots horse in the back of the head with revolver::
Mount & Blade: Warband - Huscarls and Sharpshooters rock.