Khabib will remain in the conversation for the GOAT for a long time
As for the old Gladiators, Flamma had a record of something like 21-4 with 9 draws, and Melankomas never got hit but he also never threw a punch

In the era we know more about, I'd say that the GOAT contenders are probably (in no particular order):
Royce Gracie
GSP
Silva
Matt Hughes (he looked dominant before GSP took him down, and he had a ton of pro wins)
Cruz (he was spectacular for a while)
Mighty Mouse
Fedor (he didn't see his prime in the UFC)
Bas Rutten (dominated Pancrase)
Probably some sort of Nogueira lol
And maybe Conor because he has some losses but he's had some serious opponents
I'm leaving Jones out due to getting *caught* on roids twice. Same for Reem.