I'm not much of a fan of Co-ops. They're more of a puzzle than a game. And there is the groupthink problem. I don't feel like I'm playing a game when everyone else is discussing what I should do on my turn.
That said, some of my favs are partial co-ops. BSG, Dead of Winter, Last Night on Earth, etc. Those are more either team games, or co-op with individual goals. But most of my favorites are competitive games. I get enough co-op gaming while roleplaying.