1. Team Fortress 2
Endlessly replayable, always updated and supported, there's no real reason to ever get tired of playing Team Fortress 2.
2. Mass Effect
3 times I've played this, almost to a degree of completion. Going for another run through on the PC. Why? Because it always draws me in.
3. Grand Theft Auto: Vice City
7 or 8 times I've played this. Will continue to do so as long as Tommy's adventures in Vice City remain addictive as ever.
4. Deus Ex
Only some combatability problems with Windows 7 is stopping me from having another run through.
5. Plants vs. Zombies
Played through the game again. For like the 9th or 10th time. Some "casual" game.