Star Wars: Republic Commando for the PC. I must've beaten it 20 times, and I've blown dozens of hours on the multiplayer. It was my first fun FPS that I had played (I had played games like Delta Force, but I'm not really into that), so I totally overplayed. I'll still go back every once in awhile to play it again, my favorite LucasArt game ever.
However, in terms of most time played, then definitely Team Fortress 2. Nearly 700 hours in two years, and I'm still loving it.