It's very hard to judge who's the "best" of all time is. As many people in this topic have stated, tennis has changed over the years, and to pit the best players of their respective time against each other is kind of unfair because the rules, technology, and the game itself has evolved. Now Sampras owns the record for the most grand slam titles at 14 I believe, which is an unbelieveable feat and one that places Sampras as the greatest in many people's minds. However, one could also argue that the "greatest" player should be most competent on all playing surfaces. Sampras owned the grass courts of Wimbledon in his heyday in the 90s, but he never won a championship on the clay of Roland Garros, which tarnishes the image of Sampras in some people's mind as the greatest. Now Agassi on the other hand has a career grand slam, winning all four grand slams over his lifetime, which is also a feat that hasn't been duplicated since his wife Steffi won all grand slam titles in 1988. In any case, it's been amazing to see Agassi keep his career going at the "old" age of 33 again the young gunners that are out there, and I'm sure he will always noted by many as one of the great players to play.