I got the Orange Box for Christmas last year, but I tore through everything on it in January... wow, what a ride.
HL2 & Ep 1 & 2: Epic... I never played them before. Helped make FPS games enjoyable (all it took was an engrossing story... who knew).
Portal: Portal made me feel intelligent for beating it. Granted, I did well in school and knew I had an edge, but the point is that the reward was the next level. When the solution finally dawned on you, you felt accomplished. More games need to unlock this innate desire in all of mankind. Speaking of...
Braid: See above... you have to think. It's one of those games I'm proud to have never consulted online guides to finish ('cept for those god damned stars).
Dead Space: FINELY crafted story with a level of polish seldom seen in AAA titles. The reviews below 10 were all fucking retarded, unless their complaint was a lack of multiplayer. I started my second run through before I even broke the seal of Fable 2, which I'd had for a week when I finished run # 1. People who weren't scared by this aren't scared by anything, or they're lying. Protip: Scared doesn't mean being kept awake at night after turning the game off and lamenting the dark. It means jumping when they want you to jump.
Honorable mention: Banjo demo: Wheeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee!