Emerson is both overrated and underrated. His 12 single majors were all amateur titles, won without the best pros of the time. You can see the difference in comparison to his pal and doubles partner Laver in the 60s: In the 19 tournaments, where both were in the field, Laver won 11, Emmo 2. In the 20, Emmo competed without Laver (and Rosewall, Hoad, Gimeno, Gonzales), Emmo won 10. On the other hand, overall, singles and doubles, Emmo was certainly one of the best all time. He won a record major 28 titles overall. You don't do this without being a great player. Can anyone imagine, that Davydenko would win so much, even without Federer and Nadal. In his best year 1964, Emmo had one of the best amateur years of all time, winning 3 majors and 55 matches in a row. He beat great players for his titles, including Laver, Ashe, Stolle and Newcombe. He was a fabulous big match player, winning all major finals except 3, and winning virtually all living Davis Cup matches he played. His only loss came to Santana in a dead rubber. He was in 8 winning teams for the Davis Cup - an unbroken record.
Together with Borg, he was probably the best athlete of all top players. A good sprinter in his youth, he said, that only kangoroos were faster than him, but he could play better tennis. This made him dangerous even on clay. He was probably the best coach of all time, although he never was a personal trainer. But what is not known widely, he made Navratilova the player she is, when he instructed her on her serve, while training the Boston team for the WTT in the 70s.