Wow, this is hard, here I go, there will be a shitload of honorable mentions:
1. DOOM (PC) - classic kill all gameplay
2. Tetris (multi) - most addictive puzzle game EVER!
3. American McGee's Alice in Wonterland (PC) - classic fairy tale, only told in a dark and mature way, awsome.
4. Timesplitters: Future Perfect (xbox) - What other game lets me put a monkey, duck, calamary, the Gingerbread man, a bear, a dinosaur, and Badass Robot (actual name) against each other?
5. Civilization 4 (PC) - I once lost track of time when playing this game, and I played it all night without even noticing.
Honorable mentions:
Ratchet and Clank series - can't decide on a game, one of the most unique series' out there, full of fun, humor, action, and adventure.
Jak and Daxter series - again, can't decide which game I like best,
Powerstone 2 - best multiplayer fighter EVER
Counter Strike - Scource - COD4 aint got nothin on this
Halo 3 - sure, it gets a bad rap, but I met most of my 360 friends here, and had most of my best and funniest xbox live moments.
and just so much more... like Phsyconaughts, Beyond Good and Evil, Oddworld: Strangers Wrath, and many more