I don't game as much anymore because I have too many games I don't know what to play and because I slowly lost interest.
But when I did game a lot it was:
-World of Warcraft
-Battlefield 1942
-Battlefield 2
-Medal of Honor: Allied Assault
-Counter-Strike 1.6
-Counter-Strike: Source
-Day of Defeat 1.6
I have a lot more games, but those were the ones I played nonstop at some point in time.