To answer this one we gotta first thing about what the greatest player of all time means? Does it mean..
Best level given the equipment they had?
The best level over their whole career?
Best level on any day in their career?
If we just going to say best level without looking at training knowledge and equipment, they guys like Borg and Laver not even going to be considered as their level no way reaches that of today's players. Nadal would bagel Borg if he took his best level of that era vs Nadal's of today.
But if Borg was born into the current era would be a different story for sure. Borg was an awesome athlete, and my pick for GOAT tennis player.
Even a guy like Sampras did not have the advantage of playing with the new string technology, and he himself says he would have had a huge chance at the French with poly strings.
We can't just look at number of slams either as I think Fed was real lucky and grabbed a bunch of slams when there was no one that great around (apart from nadal). It would have been great to see Fed at say 26 years old vs Novak playing like he is doing now.
If we going to say GOAT is best level achieved on any day, Djokovic probably tops the list.