Most have been said already but it's worth repeating:
Gears of War 1 & 2 - work well, but horizontal split screen. However the second one includes Horde which will keep you entertained for a while too.
Left 4 Dead - Superb fun again.
Both Rainbow Six: Vega's - benefit from being vertical split screen which works better because you can see further ahead. Definately worth it, story is linked and the second plays like an expansion pack tbh.
Halo 3 - does reduce the screen resolution and size in single Xbox splitscreen, but me and a friend still played through it. Can also go online with a guest for deathmatch etc.
And old one - EDF 2017; if you can find it - suitably fun japanese frachise that plays 'a bit' like Lost Planet. Good fun in Co-Op.
Can't think of anything else to add.