Gears of War 3 - Team Deathmatch is always good fast fun, the campaign im doing for the 3rd time (mainly for xbox acheivements) but doing it on 4player CoOP with a PUG is still fun.
Then theres Horde Mode, set up a command base, built razer wire barriers, gun turrets, plan grenades (as mines), get a massive mech suit and destroy 50 waves of increasingly bad ass locust and lambent hordes including a boss wave every 10th level which usually destroys your base haha.
Beast Mode, take on the roll of 1 of 15(ish) Horde characters and set out to destry the CoG Army by one means or another, i favour the "rip off their arm and beat them round the head with it" technique myself mwahahahaha!! i especially like to play as the Bezerker, she gets on a mean rampage!!
there are so many ribbons > Medals > acheivements you can obtain and characters to unlock, its just AWESOME!!
OR Battlefield 3, a great addition to an already great game series!! . . . thinking about getting Skyrim too but then again i probably wouldnt leave my flat until the Police turn up to have me removed for not paying rent . . . or i forgot to eat and died?