Halo for me. Having played FPS games on PC since the days of Doom and Wolfenstein I just don't see what all the fuss is about with the Halo games. They aren't bad by any stretch, but they aren't the best games in the world either. I just find so many other PC FPS games more fun than Halo.
The other problem that I have is that I started playing the games late (played Halo1 in 2005, and Halo 2 recently, both on PC; played Halo 3 on 360 in 2008) than most of the fans so I probably missed out on the initial shock of the games. They just play like slow versions of Timesplitters with some cool extras to me.