Best Co-op Games

Me and my fiance like to play games together. We've finished Gears of War and getting ready to try Rainbox Six Vegas next. Are there any other good co-op games available for the 360? A dungeon crawler would be fun but I don't think I've seen anything available yet.

Any recommendations?

I am going to upset a lot of people but this is just my personal opinion but Crackdown coop is the most fun you can have.  But if you prefer more purpose to your coop then Gears of War would be my other choice.

When I look up Crackdown on Gamespot, it says "1 Player, 2 Online". Does Crackdown have local co-op or is it just thru XBL?

IMO the best co-op game is Halo: Combat Evolved. I wish they'd remake this game with the Halo 3 engine and add live multiplayer and co-op.