My order of preference for these 10 games today:
1. Metroid Prime
2. Half-Life 2
3. Uncharted 2: Among Thieves
4. Super Mario Galaxy
5. Star Wars: Knights of the Old Republic
6. Super Smash Bros. Brawl
7. The Legend of Zelda: Twilight Princess
...and that's it because I've never played the other three games people chose.
My actual top 10 favorites from the 2000s:
1. Metroid Prime
2. Portal
3. Beyond Good & Evil
4. Okami
5. Half-Life 2
6. Yume Nikki
7. Valkyria Chronicles
8. Shadow of the Colossus
9. Psychonauts
10. Chibi-Robo!
Most culturally impactful games, IMO, by decade:
1970s: Space Invaders
1980s: Super Mario Bros.
1990s: Doom
2000s: Grand Theft Auto: San Andreas
My favorite games by decade:
1970s: Zork
1980s: The Guardian Legend
1990s: Super Metroid
2000s: Metroid Prime