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 love Spirit Island. Wide variety of options gives it near endless replay ability. The ability to widely adjust the difficulty also makes it great with a good variety of players. You can make it extremely hard for experienced players looking for a challenge or just as easily make it more casual for new players.