In my opinion it's between Federer and Laver. We all know Federer's resume, but Laver's is probably more impressive. He has eleven grand slam titles. He is one of only two male tennis players to win the single season grand slam, and he did it twice. However, he won the 1962 grand slam as an amateur and the 1969 grand slam as a professional. The reason why this is significant is that he was not allowed to compete in the grand slams during most of the 1960s (his prime) because they only allowed amateurs to compete. Laver turned pro following his 1962 season.