Fedor to me
But the GOAT is a very subjective issue, without any real criteria
For me I consider size differences, athleticism (or lack thereof), skill, competition, fight iq, overcoming adversity, even trivial things like how the individual conducts himself in and outside the cage. PED users I also frown upon, tho I can usually let one infraction slide.
At the end of the day, it's pretty much personal preference, and there are certain factors that one could argue for who they pick to have a case.