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.
I was shocked to see Federer only one the French open once. Thought he would have got a couple more before Nadal got rolling.
So there are three non-clay Grand Slams and only one clay grand slam tournament. This certainly hurt Nadal. If it were 2 clay and 2 non clay then he would have many more grand slams than the other two. Clay favors endurance, while faster courts favor big servers and less heavy top spin (flatter harder hitters). Nadal won a total of 8 non clay grand slams, including two Wimbledon's, which is the fastest surface. Federer only crossed over and won the clay grand slam once.
I can't favor Federer over Nadal (I'd call them equals more or less). But I can give the edge to Novak overall.
Federer had some time to dominate early on, before Nadal's rise and long before Novak's rise.
Nadal, all through his career, has had to face both Federer and/or Novak.
And Novak has had to deal with both Nadal and Federer all through his prime. And now Alcaraz.
I'd say
1. Novak
2. Federer and Nadal tie (slight edge to Nadal).
If another great player doesn't emerge soon, and Alcaraz can stay healthy, he could eclipse everyone as far as number of grand slam titles. A lot has to go well though.
To have three such great champions all at once (Novak, Federer and Nadal) is an anomaly I think. It's more likely than not that Alcaraz will be a lone superstar for some years. The level he plays at is extremely rare. The first three had to share titles. Not sure who Alcaraz will have to share with, other than Novak for two or three more years.