Impossible to measure that
Esp as the definition of what you call "popular" is not precise at all.
Most well-known by most people ? Most beloved ?
What I can say subjectively is that since I followed tennis (beginning of the 80s), I have the impression that two players were a kind of myth, whose name represented tennis as a whole in the ears of many people :
McEnroe and Federer
many people spoke (and still speak) about "playing like McEnroe" as a kind of myth.
Federer's name represents nearly the same now, I think, maybe not as much yet.
I guess Laver represented something like that as well, but tennis was less well-known in the world in that time.
Agassi is something completely different, Kournikova as well
Of course not, even Yannick Noah was more popular than him.
you made me laugh but I now wonder : you come from Argentina from what I know, was he popular there ?
(note that Noah was the idol of my adolescence)