While drop-in co-op is plenty of fun on its own--check out some of our best couch co-op games for plenty of recommendations on that front--this list focuses more on games where communication and collaboration are an intrinsic part of the experience. Quests in these games progress because you're both playing together, rather than despite it. These are games that feel more intimate than competitive multiplayer and more dynamic than traditional couch co-op--sitting in that sweet spot in the middle where both players are equally important.
Before you can conquer the world in Crusader Kings, you need to spend a lot of time learning the intricacies of international diplomacy and conflict, and familiarize yourself with the dozens of levers you'll need to pull to control them. But other times, complexity is something that rolls out gradually, surprising you with the layers contained within simple seeming games until it almost feels like you've discovered a secret you weren't meant to see.
After reviewing The Roottrees Are Dead, I didn't expect to sink into another compelling puzzle game that would require note-taking this soon. Blue Prince quickly occupied my brain matter with its inventive puzzle-solving.