There is no debate. The one with the most number of grand slams is GOAT, considering the fact that Big 3 have won all 4 grand slams at least once, and the fact that many reputed players of the past (Nalbandian, Davydenko, Coria, Berdych, Ferrer etc., and there are a few in line to be in the same situation like Tsonga, Nishikori, Dimitrov, Raonic) have won Olympics, Tour Finals, and Masters, but have retired without winning a grand slam. Grand slams are untouchable to ordinary players.
Ask Serena, why does she come back and try to win one more grand slam to equal Court's 24 grand slams, after she lost 4 grand slam finals in the past 2 years in an attempt to equal that record? She wants to have that grand slam record to be called as the undisputed GOAT.