Guardian of Light is a great purchase, even for single player.

The most fun I’ve had with co op so far, was actually Resident Evil 5. The controls are rough, and the storytelling is very japanese, but it’s really well paced action, easy to drop in and out + sideboob by an evil hottie.

Titan Quest is a pure Diablo clone, you either love it or you don’t. There’s also Mythos, which is free, and well executed, but becomes a monotone grind quite quickly.