In no order:
1) Resident Evil 4 - the gameplay and pacing is very addicting and satisfying, yet manages to stay challenging all the way through the game (except for Saddler).
2) Half-life 2 - the atmosphere and setting were fun to become immersed with, and combat scenarios often felt varied and fresh for most new areas I visited.
3) Halo: Combat Evolved - everything in the game felt like a new start to me; the weapons were unique, the story was intriguing, the soundtrack was awesome, the enemies were mostly smart and dangerous (even grunts were a threat when chucking grenades or firing fuel rod guns)
4) Tales of Symphonia - combat never got old, nice character development, unpredictable story development (the good kind, with lots of twist plots)
5) The Elder Scrolls IV: Oblivion - fuck, it never ends! It's just addicting side quest after addicting side quest. I love medieval fantasy themes, and it's fun to openly explore a vast world. Plus, the NPCs are buggy to the point of hilarity. XD
Honorable mention: The Chronicles of Riddick: Escape from Butcher Bay, Crackdown, The Legend of Zelda: Twilight Princess