I'd go for a big multiplayer title : Halo 3, Gears, Cod4 or UT3 along with some MS points for some of the top arcade titles. Geometry wars 2 is out in 3 weeks and Rez HD, prince of persia classic, Castlevania or Worms are all fantastic.
Mass Effect rules so insanely hard. Get that for single player.
Obviously, a Live sub is basically essential to a good 360 set up.