These are all Xbox 360. I long ago lost my patience (and free money) to upgrade my PC, which I used to love gaming on.
Mass Effect trilogy
Half-Life 2 and Episodes 1 and 2
Deus Ex: Human Revolution
Gears of War trilogy
Unreal Tournament 3
Need For Speed: Most Wanted
Call of Duty 2
I could throw various Guitar Hero/Rock Band games in too that I whiled away the hours with. Lego Star Wars for that matter too. I remember many good times with friends with these titles.