Syndicate -- can be soloed and done cooperatively. Skill-based shooting with a good progression system that lets you upgrade weapons, special abilities, and your core agent all separately to your preferred playstyle. Also some of the best sound design for weapons I've ever seen (the sniper rifle in particular). Warhammer: Space Marine -- brawling and shooting perfectly mixed, with free-form tactics necesary to survive.
What do you think the most enjoyable game for PC right now with COOP?