Im sure halo is going to be on many of your lists, but I'd like to see what other games you play. This list has single player and online multiplayer games in it, and is ranked on my enjoyment level. I'll include a brief summary of why in a few of them.
1. Halo 2 (most competitive game I have ever played)
2. Super Mario RPG (god I love this game)
3. Zelda: Ocarina of Time
4. Halo 3
5. SOCOM 3/Combined Assault (extremely fun and competitive multiplayer)
6. Paper Mario
7. Assassin's Creed 2 (epic)
8. Pokemon Red/Blue (took the nation by a storm)
9. Halo PC/CE (Assault on the Control room Co-op, need I say more)
10. Super Mario World (first game I ever played)
Honorable Mentions : Super Mario 64, Super Smash bros Melee, Goldeneye 007, Mario Kart 64, Crash Bash, Crash bandicoot warped, Gran Turismo 3, Need for speed most wanted, Jak 2, Skyrim, Paper Mario and the TenThousand Year door, Mario Party 2, Age of Empires II, Donkey Kong 64, Zelda Majora's mask, Tibia, Major League Baseball featuring Ken Griffey Jr.,