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.
US Downloadable Games - Adventures To Go! ($14.99) - Cho Aniki Zero ($19.99) - Rengoku: Tower Of Purgatory ($11.99) - Rengoku II: The Stairway to H.E.A.V.E.N. ($11.99) PSP minis - Alien Zombie Death ($3.99)