Co-op specific levels

This may be an already existing thread, but I didn't find one in a quick, before work search. So quick question, what are some good co-op only/designed specifically for co-op wads? I know there's tons of great wads that feature co-op compatibility, but I'd like to find some that are more geared towards good co-op play than just as an afterthought. Maybe even some with co-op only situations/puzzles/whatever.

Thanks in advance

Drown in Blood I think, I also think it's supposed to be played with a fast weapons addon :P

I'm curious too. Been playing Sunlust w/ friends lately and it's fun. Would be cool to find some other modern maps with co-op in mind.

Pretty much all the slaughter maps.