I'll second what the others have said, and add The Orange Box. The Assassin's Creed games are good too. The Forza games are good if you like racing games. EDIT: Better idea. Just go buy all the greatest hit games.
Halo: Combat Evolved Halo 2 Halo 3 Call Of Duty 2 Call Of Duty 4 Modern Warfare 2 The Force Unleashed Dark Void Castle Crashers Call Of Duty Classic Classic Batttlefield: Bad Company
^^^ Not a bad list there Mecha, but the thought occurs to me that when abbreviated COD and MW2, does Tkemert20 have any idea what you could be referencing? He probably does, but that thought is crossing my mind. Anywho, I'll try and avoid being redundant by post the same titles over and over. Mass Effect 2 -make sure you play the 1st one too (can find for $20 now), makes playing ME2 all that more neato Mercenaries 2 - While the game might have not be as great as the first one, it's still fun. Also as a bonus, it's cheap now, like $20 or less now. FEAR 1 & 2 - Might be a bit of a biased opinion, but I'm totally gay for these shooters. For this game being a horror game, there're still plenty of intense shootouts. Should also find a copy of FEAR 1 for pretty cheap. The 360 version of FEAR 1 isnt bad, but if you can, I'd suggest going for PC if you got the system to run it. Ghost Recon Advanced Warfighter 2: Best Tom Clancy title on the 360 (Splinter Cell Conviction pending). Also can find for cheap Ace Combat 6: If you can get past the redicious sounding disney dialogue, you'll have yourself a pretty good air combat game. I have some gripes about the multiplayer however. Some MP dogfights are about as bad as a Street Fighter 2 match of Ryu & Ken doing hadoukens non-stop. You'll get matches where people will either fly F-22's or Su-47's (top tier craft), fly in a non stop loop, fire missiles and hope for the best. I'd would suggest checking out Tom Clancys: HAWX for air combat as well, but check the demo for that first, it does things a little different from your typical air combat game. I think it does have the better multiplayer air combat when compared to Ace Combat 6. Shadow Complex - It's a Xbox live market place game, so you wont find a CD for it. It's played like Super Metroid, but with guns.... But Metroid has guns.... Just get it! Thats all for now.