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.
Challengers Pack with Pre-order Pre-order "Ultra Street Fighter IV" and receive the Challengers Pack which includes five additional costumes for Elena, Hugo, Posion, Decapre, and Rolento. Ultra Street Fighter IV - Xbox 360 (The W at Amazon)