Well, as far as what he has accomplished inside the octagon, he is #1 if we try to go by the most logical criteria. The only fight officially recognized as unfair due to PEDs was the DC II... So on no contest. He is 29-0 officially. 28-1 if you consider his DQ because of upwards elbow when the opponent was already defeated and the judge wouldn't stop... But common sense is 29-0. Only Khabib has it, but most of half of Khabib's fights were not in the UFC. 3 of Khabib were title defenses. JJ had 26 fights in the UFC and defended his title 13 or 14 times (roughly the amount of fights Khabib has in the UFC) and is a double champion.
The others champ champ have not a 29-0 record either... You can't fight against numbers. But if GOAT includes things non fighting related, like, the impact the fighter has over aspiring fighters, the positive influence they give, then ok, sure. But if GOAT is to fighting only, then there's no way around that.