Firstly I have to state I am a Federer and Nadal fan, and this is a very unbiased thread, but just what I have observed over the years. I would still say Federer is the greatest player followed by Nadal second, then either Djokovic or Sampras number 3.
Federer had a trouble free start to his grand slam wins, there weren't many notable opponents in the early years. It was a retiring Agassi, and Roddick and few others which weren't of the highest level. Nadal troubled Federer from the very first game. When Nadal and Djokovic started to emerge as multiple grand slam winners, Federer struggled. Federer also enjoyed a very weak era last year.
Djokovic hugely benefited from the end of Federer's career where his form was low, and the same with Nadal in the last few years. Nadal was obviously going to drop off quicker due to his style of play and the impact the body takes. So Djokovic enjoyed a free ride over the last few years, before 2017, and at one point held all four grand slams I believe.
Nadal had it very tough. He was playing on the men's tour when he was a child. He had to compete against who most people consider the greatest player of all time Roger Federer. Nadal was number 2 for so many years, but him and his uncle worked out a way to beat Federer at his own home, and he did so in Wimbledon 2008. Federer was untouchable on grass from 2003 to 2009, and was still in his greatest form in 2008 and Nadal had to battle on his weakest surface to beat him. He then had to compete with Prime Djokovic, against who I would probably consider the third greatest player of all time. So to think he had to compete with the greatest and third greatest players in history in their primes.
