I think the better question is who maintains a strong level of play consistently. Players like Tilden, Rosewall, Borg, Pancho Gonzalez, Jack Kramer, Jimmy Connors, Ivan Lendl, Riggs, Budge, Nusslein are among the players of the past who kept a very high level almost all the time. In recent times of course you would have to say Nadal and Federer. And in recent years you would add Novak Djokovic to that list.
Ellsworth Vines was known to be erratic at times as an amateur but in the pros his record indicated that his level of play was consistently high.