I can only refer to the games that are also on xbox 360.
As mentioned Bioshock 1 & 2 were great if you like horror survival games, Fallout 3 & New Vegas are just friggin great epic RPG/shooter crossovers (mainly RPG) & Borderlands is all kinds of fun. The best game across platforms has got to be Red Dead Redemption though.
For something a little different and family appropriate, Ghostbusters is pretty great too. I bet you could probably get it pretty cheap.
Astute people may have noticed I changed up my signature recently. There's a link to a blog now. Why, you can Click Here (basisgames.blogspot.com) to see it. My brother-in-law and I have started a video games blog as a little hobby.