My GOAT rankings:
1- Khabib Nurmagomedov: 29-0 undefeaded and undisputed in an era where Lightweight is the best and most stacked division in the UFC. Never lost a round (apart from one he took off standing up with Conor, and it was very very close), never been dropped, never been cut. Absolute dominance in grappling, wrestling and submission game. His striking style (even though not the best in the division) complement his wrestling perfectly.
2- George St. Pierre: The most dominant Welterweight champion who avenged his 2 losses. Took 4 years off, returned to win the middleweight championship. His world class MMA skillset is well rounded and complete.
3- Jon Jones: Most title defenses. The youngest champion in UFC history. Cleared out his division for two generations of fighters. Had only 3 close fights in his career (Gustaffson 1, Thiago Santos, and Dominick Reyes). His PED use will always be an issue in a physical-demanding sport like MMA.
-----------------------------------------------------------------------
Of course, as always, GOAT is subjective and others could disagree with my argument there. I can understand anyone who changes the positions of these 3, but for me, the GOAT conversation should always have these 3 champions on top.