I'm looking for split screen co-op to play with my friend too. So far I've played game listed below.
Gear of War 1/2 - 10/10 The best co-op game I've ever play
Army of Two 1 - 8/10 Acceptable, haven't get 2 yet
Lost planet 2 - 2/10 Sucks, split screen with a huge map beside, make the screen looks damn small, not enjoyable for co-op
Call of Duty World War 2 - 5/10 So so, don't really like world war 2 guns
Call of Duty Modern Warface 2 - 6/10 normal but short, no split screen for story mode, only for a mode that you need to finish its task only (forgotten the name)
Kane n Lynch Dog Days 2 - NA/10 Just got it not long ago, played 1st chapter in myself, then busy with Mafia 2, not yet try the split screen yet
Marvel Ultimate Alliance 2 - 6.5/10 Fun to play, not split screen, you see all the charaters in the same screen
Will get Splinter Cell Conviction, Terminator Salvation in the future, heard that it got split screen co-op as well. Need to get a bigger LCD TV as well, 32" for split screen is too small.