Project Reality:
Best mod ever. I don't even know how to describe it, all I can say is extremely great or something like that.
As the name says it aims at realism, so teamwork is required. But if you don't play as a team, don't expect to win and don't expect to have the full experience, it's all about teamwork. If you don't play in a squad you are going to be running to where the action is for half of the round. Also if you choose to try this I highly suggest get a microphone and start using it.