Well for a 360 COD6 is a no brainer. But other than that Gears of War 2 is a great game to play with friends. And if you like the whole taking cover thing Rainbow Six Vegas 2 is pretty much like MW2 but more tactical and you can take cover. You can also mod your guns to have al sorts of different kinds of scopes and sights.
And if you like Borderlands,
Mass effect 2 is coming out at the end of the month. You should look it up. I enjoyed the first one.