Resident Evil 5 - it's really fun when you play it with a pal, but well... it wasn't very Resident Evily or scary at all, which is why everyone else on the internet loathes it.

It's irrational, has more than a few flaws and is short, but it still bloody well beats co-oping in Army of Two or any other co-op game available at the moment. Incidentally, they seem to be planning co-op campaigns for a lot of upcoming games (FEAR 3, Dead Space 2 --) and it'll be interesting to see if they stick with it and how they'll execute it, in that case.
Also, Army of Two: 40th day with VERTICAL splitscreen? That's not even funny.