I haven't had time to play my xbox in a long time but the must play single player games are:

Fallout (3 and Vegas) (and Oblivion)- Amazing western first person rpgs Oblivion is old but probably my favorite game ever

Mass Effect (1 and 2)- Space Voyaging 3rd person shooter rpg

Bioshock (1 and 2)- Underwater "atlantis' gone corrupt with drugs and psychos fps

Assassins Creed I II and Brotherhood are supposed to be decent too but idk.

And for multiplayer Black Ops is great but Gears of War 2 is my personal favorite.

(Haven't gotten to play most of the sequels to the single player games yet, but I own them and wouldn't doubt they are good.)