1) Deus Ex
Best game ever. Period.
PS, if anyone played this, if I told you my handle, you'd know who I am. I was the best in the world for a couple of years. (No joke )

2) World of Warcraft
What dude above me said. I was in a very high end guild at one point. Being one of the best at a game so many play is very satisfying.

3) Call of Duty 4
Innovative. Balanced. Beautiful. Great competitive scene.

4) Team Fortess 2
Blame GetThere1Time for my love of this game. He's a dicksucker.

5) Medal of Honor: Allied Assault
It's so simple. It's so balanced. Was also my first "clan" based competitive game. Many, many good memories. Also got me a trip to Scotland to hang out with the clan I played with back in those days. Imagine the look on my mother's face when I told her a bunch of 20+ year old guys are gonna fly me to the other side of the world to meet them. (I was 16. ) She was reluctant, but god damnit, it was fun!