5. Star Wars Jedi Knight: Jedi Academy - I still play this game a LOT. It's so damn fun to go evil.
4. World of Warcraft - Taken a dive in the last years, but it's still fun and addicting.
3. SimCity 4 - Win on so many levels. Much harder to fail miserably than the previous in the series.
2. Half-Life 2 - The Source engine is a piece of wonder and the game is amazing. Nuff said.
1. Freelancer - It's FL ferchristssakes. Add in Disco and it has the most replay value of all five games here.
Honourable mentions include SPORE, Sims 2, JK2, Counter-Strike: Source, and Medal of Honor: Allied Assault.