Dr Ian McHale has done a mathematical calculation of over 20000 Grand Slam matches involving over 1,000 players with a “highly sophisticated computer”. Dr McHale looked at Grand Slam competitive records of 1,163 players in 725,754 games and 20,640 matches. He then identify the year of maximum strength for each player. He has come to the conclusion that a 1976 Jimmy Connors was the best player of all time. Second was Borg followed by Agassi then Federer.
Personally I am more skeptical of this. Firstly during the different decades the study doesn't take into account playing styles. Ok Jimmy Conors was a incredible player during his time but would he still be as successful if he applied his exact playing style into today’s game of fast serves and hard hitting? The other issue is this study takes into account grand slams but as far as I can tell no other tournaments. To answer the question who is the greatest player of al time I would answer by saying nobody. Unless there is a time machine which put all of the great tennis players at their peak into one single tournament I would argue that we can’t say. It right to say greatest player at that time or decade but not of all time.
Source - http://www.salford.ac.uk/old/news/connors-is-greatest-tennis-player-of-open-era,-says-top-sports-statistician2