Team Fortress 2 definitely. I have nearly 3K hours in the game. It's just so addicting and fun and I never get tired of the strategy and advanced tricks involved with the classes.
Me 2. What class do you play the most (top 3)
Mine are
1: pyro, as a pybro using the old shotgun and flare gun
2: demoman, sticky spam and demoknight
3: soldier for rocket jumping
4: Medic, because its fun/ nice to hear a thankyou in the game especially through voice chat