I would be curious about your favorite coop games. I often find them a big lacking, often being a bit unbalanced. An example is Battle for Hogwarts, where one game can be a walk in the park and another absolutely impossible and rarely is there a balanced match.
I found the game Castle Panic recently. The rules are fairly simple and didn’t take too long to understand which was a hit with my friends. The element of chaos and the near-constant pressure until the end was fun. If it is too easy, you can handicap yourself in one or more ways which will increase the difficulty.