Just plain fun? I hate to say it, but Halo really wins out for me, then. Rocket launcher deathmatches, Snipers, OP pistols, that sense of excitement when you came face to face with a pair of hunters... All of it combined to make a franchise that's been fun, despite the amount of hatred that it gets.
I remember playing the halo trial in a sixteen person LAN match during science classes at school, and damn, it was fun. No matter how impressive, thought provoking, disturbing or twitch based other recent FPS games have been, I don't think that anything else really comes close. What they manage is *excitement*, or immersion, but not much fun.
That said, I always enjoy playing TF2, so there is that.
/edit
Typed this from my phone, so I apologize for the numerous mistakes I've made in writing this.