My fav FPS is probably the first one I ever played Delta Force published by NovaLogic. I worked for an ISP in Houston back then and we played the shit out of that game
During all the LANs I ran, I would say that we had the most fun with Unreal Tournament (and many many mods / mutators) and Sven Co-Op, the Half Life Mod...
My 2 all time favorite shooters are Unreal (the original) and Doom 3. I haven't played any of the recent ones, though. I pretty much stick with WoW these days because I'm getting too old to compete with the kids in today's games. As I get older, my reaction time gets slower, which isn't good for fps games.