I just finished Half-Life the other day. Definitely, a favorite. On the other end of the FPS concept, Serious Sam is something I desperately want to try, despite the fact that I haven't really been into fast-paced shooters all that much. Halo: Combat Evolved, I had a blast with, and I'm having the time of my life with Doom (neither of which I've actually finished, yet)! The 007 franchise has had it's fair share of good games, and I really rather like 007: NightFire. There are three versions of this game (PC, Console and Handheld), and I'm only truly familiar with the PC version, so, I really couldn't tell you how each one holds up when stacked against each other. (Though, I suspect the handheld version is inferior, and the console version might be the better game.)
Left 4 Dead is a nice, simple, adrenaline rush, Team Fortress 2 is a wonderful breath of fresh air from all the lifeless F-Bombsquad marines that run around in gray environments looking for reasons to be sad and treat people like dirt all the time, in the most completely out-of-context way possible. I hear Ghost Recon is good, Counter-Strike was an interesting twist and the human race still has yet to catch up with Ricochet. At least... That's what Valve says. ...In Valve Time!