I did forget Nadal. He's an all time great. Is there a consensus that he is the second best player of all time? By the measure of majors, I guess you have to give it to him over Federer. Is Sampras 4th best of all time then? Are majors the best way to measure? I like tennis but I'm not an expert, by any means.
I think Novak is the greatest because he is the most complete player on all surfaces collectively (see below for more on that). I think Federer is 2 because of time at #1 and general dominance on both grass and hard court. Nadal has 22 grand slams, but 14 were at the French (which is also why both Novak and Roger didn't win more there). Nadal is much more a specialist than the other 2 so I'd have him 3. After those 3 I think Borg, Laver, and Sampras are all close together and very difficult to separate given era.
Also, more support for Novak, he is the only man to be the reigning champion at all 4 grand slams with the 3 different surfaces (laver played on grass and clay only). He is the only man to win all 4 majors at least three times. He is also the only man to win all 9 masters tournaments in his career, which he has done twice.
Borg ? Connors and McEnroe were every bit his equal.
perhaps on hard courts, but Borg won 6 french and 5 straight at wimbledon. In fact the only man to ever beat him at the French is Adriano Panatta who beat him in the 4th round in 73 and the quarters in 76. Every other French Open Borg entered, he won. McEnroe did end Borg's 41 match win streak at Wimbledon by beating him in the 81 Final. After losing to McEnroe again at the US Open Final in 81, he walked off the court and basically retired (he played like 4 lesser tournaments until he tried to come back in 91). He was 26 and still dominating on clay and grass, but walked away in his prime.
Because Borg was so dominant on grass and clay, and won so much, I think he is ahead of Connors and McEnroe. Those 11 grand slam titles are still 5th best all time in the open era, behind only Djokovic, Nadal, Federer, and Sampras (Emerson has 12 and Laver has 6 pre and 5 in open for 11 total). After Borg, 8 is the next most in the open era from Connors, Lendl, and Agassi. McEnroe is tied with Wilander with 7.