At the minute my must play game is Half Life 2 from The Orange Box. Although, in saying that, I don't know what games you're into; for an FPS it's not the most intense when it comes to the action. For that, I'd suggest Call of Duty 4 for a varied single player game and a ridiculously well designed multiplayer game
Ah, you already have CoD4! Hmm, Rainbow Six Vegas?