With friends (people I know outside of xbox live), it's GTA4. Free mode, team deathmatch or even racing is just good, clean, chaotic fun. Nothing beats landing a chopper on a skyscraper and doing synchronized suicides with your buddies-- especially if one of them fails to jump out of the chopper as it topples over the edge of the building
If none of my friends are on, then usually I'll fire up CoD4 or Gears.