Well to be honest i totally play online on XBL and hence havent gone through much campaigns, Anywayz i will recommend you to buy GearsOfWar2, COD5, Prototype, SplinterCellDoubleAgent, PrinceOfPersia. Exclusive titles arent that hit like PS3 ones (MGS4 and KillZone2) but still they are famous due to their Stable Online gameplay. You may go for Halo3 , Left4Dead if you are going online in the near future. I have told you mostly shooter games but there are lot more in other genres. Let me know if you need my further opinion on remaining genres