I've wondered that myself. The statistics seem to suggest Laver by a hair, or co-equal number 1.
"Laver won 11 tournaments and Rosewall 10; Laver beat Rosewall 15-4 in head-to-head matches."
"A point system for 19 pro tournaments (excluding at least 10 other tournaments) also resulted in Rosewall being No. 1 to Laver's No. 2 but that system granted each tournament the same points and then was unfair to the big events where Laver was superior to Rosewall : Laver beat Rosewall & Gonzales in U.S. Pro; Laver again beat Rosewall in Wembley Pro; Rosewall beat Laver in French Pro."
Maybe it is this "unfair" point system.?
The smart man thinks he knows a lot; the wise man is aware that he knows little.