Definitely the paper cups provided by the club. The propensity of supposedly educated and high earning individuals to leave their garbage around is nauseating.
If you're talking about things left behind that are not just trash, I'm guessing vibration dampeners, jackets/sweaters and racquets, in that order. Presumably the racquets are typically left by kids in group lessons. There always seems to be one on the court when I come on after a kids' class - not sure why, but the last activity in many of the kids' classes seems to be without sticks.