Definitely:
Fallout 3 - My Favorite game of all time. Great story and if you got a big TV you'll feel like your in the game, legit.
Call of Duty: Modern Warfare 2 - It's kinda just a must have. It has a really awesome campaign. Good storyline. I personally enjoy the multiplayer just because a lot of people play it, but be aware that you're gonna run into some really annoying little kids playing.
FEAR - Great for PC, but it's also out for xbox. This game will scare you. It freaked me out when I played.
Half Life 2 & Portal (Orange Box) - I must agree with the posts before me. Half life 2 is a great game. I played it on PC with maxed out graphics and it was sick. Portal is mas fun, and trying to beat it can be difficult at times. But if a game doesn't have a challenge then what's the point of playing it?