I'll try to go with one for each genre.
Battlefront 2: FPS. I grew up on this, and its still fun as I'll get out.
Kotor 2: RPG. A lot of people prefer 1, but I enjoy the mechanics of 2 better.
JK2: Jedi Outcast: Action. Great level design, satisfying combat, and a good story.
(Non-Star Wars alternatives)
Goldeneye: FPS. (The Wii version, I guess) Nice controls, fun stealth.
Deus Ex: HR: RPG. Only other RPG I've really gotten into.
Bioshock: Action. Same as before. Good level design, satisfying combat, great atmosphere.
Last 2:
Team Fortress 2: Multiplayer. Hats.
Portal: Puzzle. Portal 2 was good, but less of a true puzzle game.