I'd say as of right now (no particular order after 1st one)
Half Life 2- I just love this game so much. It's the game I played that made me want to do something in the industry.
Left 4 Dead- I love the co-op, the character design, and the Survivors
Portal- Okay, most of my favorite games are Valve games. But do I need to explain this one?
Fallout 3- I don't want to know how many hours of my life have gone to this game
Mass Effect 2- I never realize how many hours have gone by when I play this game because I'm so fully immersed.