[Lifetime]

I'd have to say, get Jedi Knight 2. Awesome game, especially when it comes down to lightsaber combat. Unreal Tournament and Quake 3 are getting old, but are still good, most people prefer UT to Q3, because Q3 seems like it was made for people who were weaned on the earlier Quake games. Q3 caters naturally to the Quake 2 crowd, people who want an insanely fast, skill-driven experience, essentially the best deathmatch possible, and the most twitch-reflex game you've ever played. Nothing else comes close. UT however, along with all the mods, mutators and powerups is the most flexible and diverse FPS around, catering to every kind of experience, from Tactical Combat to CTF to Deathmatch. Some of the mods in UT are really pretty cool.
Then there's AVP2, which is scary as hell until you realise how to properly play the game, in which case its scary adrenalin-filled fun. Nothing like playing with other human beings, since nothing captures the mind of a wall-crawling Alien more than a very demented human being. Serious Sam 1 & 2 are excellent stress-relief games, Red Faction is an average FPS with a really cool game mechanic, Max Payne is just oozing class.. You're really spoiled for choice if you've got the specs to run these babies.