No one has ever done something ridiculous like winning 4 calendar Slams in a row so there will always be debate. Slam count is the most important criterium of course, but not necessarily decisive... Williams less than Court, Borg and Laver less than Emerson eg. And there is the question of strength of competition on each surface, which doesn't necessarily balance out with every generation. But there is a 'name recognition factor' which is probably the ultimate decider, at least in the public eye. And on that score, I think there's little doubt that the 'name' of Federer commands global supremacy when the subject of tennis is raised, with every other player on a lower level. Sells more tickets, garners more column inches and, most spectacularly, earns more crowd support even when facing opposition in their home country. Artistry, style and elegance has as much bearing on this 'name' factor as actual wins and titles. So even if Nadal and Djokovic surpass his numbers thanks to lesser opposition form the 90s Gen, I think the 'tennis public', outside of specialist forums like this, will continue to reverence Roger Federer as the GOAT.