Who is the Greatest male Tennis player of all time?
Here are few things to consider to come up with the player. 
1. Player who has won most grand slams
2. Player who spent most time as world #1
3. Player who has won grand slams on all three surfaces (Hard, Clay, Grass)
4. Player who has won most year end ATP finals and also Olympics
5. Head to head wins between players

Based on this criteria, the greatest male tennis player of all time is Novak Djokovic. There are two other male players who are in contention: Roger Federer and Rafael Nadal. Novak and Nadal both have won 22 grand slams each. All three have a favorite surface (Grass for Roger, Clay for Nadal and Hard for Djokovic) but they won grand slams on all surfaces. Djokovic has the record of spending most time as world #1. He also has a better head-to-head record over Nadal and Federer. Djokovic and Roger share the record of winning most year end ATP Finals. Roger Federer kickstarted the golden age of Tennis and he is extremely popular for his skill, talent and grace with which he used to play. Roger won Wimbledon the highest number of times among all players. Nadal won French Open 14 times which is a record that potentially may never ever be broken. History will remember all three players for their contribution to Tennis and a lot of tennis fans are lucky to have been part of the Golden age of tennis.