Your a lucky boy. I Co op a lot. It may take me a while to think of all the best co ops but I know a few off the bat. I won't name any generic shooters with decent co-op like GoW because you likely know em already.
Here's a few gems.
Tomb Raider GoL. Co-op isometric action 2 stick puzzler. Poorish story but great co-op.
Castle Crasher/DD:neon or any other side scroller 360 beat em up. Most are great just avoid the other double dragon game as it is shoddy. There's also a few arcade/saturn side scrollers that snook in.
Pay day 2. Great team based shooter. It isn't pvp it's full co-op cash grabbing action. 360 version is a bit crappy due to poor dev support but there is still hope for dlc/patches
Unreal Tournement 3. I'm pretty sure its fully single player can be played co-op It's great if you missed it and should be real cheap where you find it.
Tales of Vesperia. probably the only decent co-op rpg on the 360. Main player does all game decisions but upto 3 friends can take up arms in the battles. Offline only co-op unlike the ps3 tales of.