Jack Kramer is certainly a GOAT contender. As soon as WW2 ended, he became the dominant amateur player, turned professional and it didn't take him too long to get to grips with Bobby Riggs (the best professional player), and Kramer was soon beating Riggs at will with a relentless serve and volley game.
As an amateur, he won 2 US Championships and a Wimbledon title. As a professional, he won the 2 big tournaments with the US Pro at Forest Hills in 1948 and the Wembley Pro in 1949. More importantly, he was so brilliant on the world pro tours, thrashing players like Riggs in 1948, Gonzales in 1950, Segura in 1951, and edging a closely fought tour against a brilliant Sedgman in 1953. Injuries then put an end to his career, apart from those 1957 tours against Hoad and Rosewall.
Kramer was also legendary in other parts of tennis, on the promotional side, equipment side, commentator, and trade unionist with the ATP. He has to be one of the most influential figures in the history of men's tennis.