If u noticed great tennish player from Ivan lendle to Roger fedrer, player who are dominant in grass court are not same in clay court and vice versa. Lendle won every grand slam but French open. Boris becker won except French open. However there are players who had moderate success winning on both form of court. Bjon borg was the one of the greatest player who dominated the both French and wimbledon. Mats willander almost won grandslam only to lose in wimbledon in quarterfinal to miloslav mecir. Who is the greatest player, its kinda subjective. But my vote will go for Bjon borg