The greatest era would be the late 1950's in the pro game, beginning with Hoad's signing after the 1957 Wimbledon, and ending with the January 1960 Kooyong pro (this was actually the last event of the 1959 season). After this, Hoad refused to play the American championship tour, and Gonzales withdrew from tournament play, and there was insufficient play to determine a world champion for three years. Laver's turning pro in 1963 did not infuse new life into the pro game, and the pros stopped appearing in major venues until Open tennis arrived in 1968 (the 1967 Wimbledon Pro was arranged after the All-England Club had authorized open play for 1968).
In the extended 1959 season, there were eight full-time pros (plus Mervyn Rose and Rex Hartwig) who played a full schedule of tournaments world-wide, with each player winning at least one important event (Hoad won six tournaments (including Forest Hills and Kooyong 1960), Gonzales five, Rosewall two, Sedgman two (Kooyong 1959, Grand Prix de Europe), Trabert won at Roland Garros (with Hoad beating Rosewall for third place there), Cooper won the Slazenger in Britain, Anderson won Wembley, Segura won Tel Aviv). In addition, Gonzales edged out victory in the American championship tour, despite dropping his series with Hoad 15 to 13 (and splitting his sixteen tournament matches with Hoad at 8 to 8, for a 23 to 21 edge for Hoad on the year.)
This was the most complete year in pro tennis ever.
If we consider more than 4 players, then I agree that 1959 has to be rated among the best ever.Since the inception of open tennis, I´d pick:
1971 (Ashe,Laver,Newcombe,Roche,Rosewall,Kodes,Nastase,Smith,Okker,Gimeno making what is definitley the best top 10 ever)
1975
(Borg,Connors,Newcombe,Laver,Rosewall,Ashe,Vilas,Orantes,Panatta and Ramorez or Dibbs )
1979
(Borg,Connors,Mc Enroe,Gerulaitis,Vilas,Tanner,Pecci,Solomon,Higueras and Dibbs)
1981
(Borg,Mc Enroe,Lendl,Connors,Gerulaitis,Clerc,Vilas,Tanner,Kriek and Mayer9
1985
(Mc Enroe,Lendl,Connors,Wilander,Becker,Edberg,Noah,Cash,Gomez,Mecir/Kriek/Curren)
1994
(Sampas,Agassi,Edberg,Courier,Becker,Chang,Ivanisevic,Bruguera,Krajickek and Stich)