Space Engineers, with lesser graphics but makes it up with a stable multiplayer that doesnt bug out in places every 30 minutes. Plus its still alpha! :)

Cutting apart thousands of rats in half, in the middle of a apocalypse. A simple but hard to master co-op game.

Warhammer 40K?.... check. A better co-op survival arena then Gear of Wars? check... Challenging? check! A old but great co-op game.

Hard to setup and maintain as server, do to many bugs. But once your able to get things stable, its probably one of the best co-op games.

It was stable, comes with a lot of micro/major updates and withstanded 50+ hours of gameplay within a group that likes to variate in games... a lot :)