Who is the greatest male tennis player of all time?

Roger Federer, Rafael Nadal, and Novak Djokovic (aka the Big 3 of tennis) are all regarded as the greatest male tennis players of all time. If the criteria for greatest of all time is based on the number of grand slams won, then that title is shared by Nadal and Djokovic at 22 majors. However, many fans, pundits, and sports analysts consider Federer to be the greatest of all time due to his longevity, success, and ability to attract fans to the game, and humanitarian impact off the court. Ultimately this is a very subjective question and is difficult to answer without first defining the criteria for greatness in tennis.