MW2 or MW3, they both have spec ops, which is fun.
Any Army of Two game
Theres an indie game on xbox live marketplace called zombie estate, and the second one was just released, its only 80 msp ($1.00)
Edit: Splinter Cell: Conviction was fun
Zombie estate 2 is incredibly fun. I recommend it over #1.
Lost planet 2 is a fun coop game, earth defense force insect armageddon is a good one, the halo games are fun on campaign/multiplayer. There's an indie game "20 games to play with your mates" or something like that, some of the minigames are p fun. Murder miners is pretty good, minecraft and terraria are fun.