Most competitive games could be considered co-op experiences. Unless it's deathmatch, you usually are on a team in which you work together.
Personally, I would have to go with co-op, simply because it's more fun to play with friends than against them. But the best is playing with friends against other people.
Personally, I would have to go with co-op, simply because it's more fun to play with friends than against them. But the best is playing with friends against other people.