I think there is an argument for any of them to be the greatest.
Nadal’s clay record is absurd and probably the highest peak level tennis has ever seen. 14 RGs is insanity.
Djokovic has the actual numbers, best longevity and best body that have all sped him to remain physically good and healthy for such a long period of time. His dominance was spread out on a longer period than Federer or Nadal. That’s a big plus on him.
Federer domination in 2004-07 for 4 consecutive years is unheard of as well. It was possibly the highest peak period of domination I’ve seen a tennis player have. He set the bar insanely high during that period. So many consecutive records like GS finals, GS SFs etc etc.
On another hand, one can argue that Djokovic had the easiest competition because there was no potential ATG coming up from the newer generation until Alcaraz came on so he could really rack up more slams, especially since he’s the youngest of the big 3. Federer is like 6 years older and look the younger ATG will have the edge.
Federer had a head start but let’s be real, they played more times when they were in their 30s and as the older ATG, Federer was always going to be a at a disadvantage since age is a pretty critical factor in tennis as movement and speed is a pretty big factor.
Federer has 2 ATGs competing with him, Djokovic as the youngest ATG had no one else, which made it easier for him to rack up slams after 2018. Federer was the one set the bar and he was always the one defending the lead. And he defended it until 2022 which is surreal. In a hypothetical scenario where Federer and Djokovic swapped ages, I think Federer would have more slams. But I can’t prove that obviously.
With that said, history won’t see it that way. History will see Djokovic as the one with the most slams, the one with the best numbers. Hence, the GOAT. And I can live with that now.