Well, Halo 3 is obvious and has already been mentioned. And you should really consider renting Gears of War (not buying though, unless you really like the multiplayer (and I mean you have to think it's the highest and most perfect form of multiplayer ever created)).
Oblivion and Mass Effect as well, assuming you don't have them on the PC. Assassin's Creed fits into that group as well.
And there's the quickly encroaching releases of Fable 2 and Gears of War 2.
CoD 4's a good rent or buy too but I assume a self-described "hardcore PC gamer" such as yourself already has that.
Those were 360 suggestions by the way.
Oblivion and Mass Effect as well, assuming you don't have them on the PC. Assassin's Creed fits into that group as well.
And there's the quickly encroaching releases of Fable 2 and Gears of War 2.
CoD 4's a good rent or buy too but I assume a self-described "hardcore PC gamer" such as yourself already has that.
Those were 360 suggestions by the way.