I think who is best tennis player of all time is a very subjective question....but it has to be among three Rod Laver, Bjorn Borg and Roger Federer

Bjorn Borg, apart from his game, was the guy who popularize tennis in the 70's and 80's. And he had the "game" to win both in grass and clay. He won 6 french open and 5 wimbledon.

Federer would have easily won 3 french open hadnt there been Rafael Nadal - probably the greatest player to play in clay court now. As someone earlier pointed out, he has the complete game. He can "play" and win in clay.

And because of this last point, Pete Sampras wont be considered in the same breadth. Sampras was never in a position to win a French Open. His best performance in FO is a Semi Final berth in 1996 - this guy wasnt comfortable in clay court. This is subjective but if Federer was playing in 90's and there wasnt a guy named "Nadal" he would easily have won French open couple of times. Just consider the player who won French open in 90's

1990 - Andreas Gomez, 1991 - 1992 - Jim Courier, 1993-94- Sergi Brurgera, 1995 - Thomas Muster, 1996 - Yevgeny Kafelnikov, 1997-Gustavo Kuerten, 1998- Andre Agassi, 1999-00- Gustavo Kuerten

Although Agassi, Kuerten and Courier are great names, there isnt a single player here in the pedigree of "Nadal" in clay - meaning Sampras wasn't just good enought in clay even against medicore player. While, I believe Federer would have won French Open if he hadnt come across Nadal - and he might in the future, he has the game to win it.

My all time favorite is Agassi. He along with Borg popularized tennis to unprecedented level and were "cult" figure among the masses.