Since evaluating things like P4P rankings and GOATness is fairly arbitrary, here are five criteria to evaluate fighters in relation to one another.
Record - This has a few factors. The most obvious one is looking at the number of wins and losses, the quality of competition relative to a given epoch and division depth, and also absolute terms (who overall beat the best quality competition, regardless of quantity). We can evaluate competition in terms of how many ranked wins one had, champions or former champions, P4P ranked fighters, what was the combined record of opponents, and whether the opponents were in their prime or not, among other things.
Also we look at who the fighter has lost to, and whether the losses were avenged or not, and the conditions under which these losses took place, e.g. Fedor's loss to TK was a fluke cut, just as Jones' DQ loss; GSP's loss to Serra was more significant, though he avenged it decisively;, Cain's loss to Werdum was definitively impactful... This category also includes titles won and title defenses: how many titles won, how many title match wins, how many weight classes? How many title defenses did they have? This factor is complicated by the introduction of primes and point in career: should Penn's record after his reign and prime count as significantly? Should pre-prime records be compounded only? Since it is hard to determine with any fair sense just when a prime ends and begins, this makes things difficult and always controversial (e.g. was Fedor past his prime when he lost to Werdum?).
Dominance: How did the fights themselves go? How consistently did the fighter come out on top in a decisive manner? How far ahead of everyone else was he/she, record aside? Khabib is a good example of a fighter that has very few title defenses relative to other fighters, so he does not excel on paper in terms of title reign although he was undefeated. But he was so overwhelmingly dominant in the fights themselves that he is taken to be one of the GOATS. Someone like Jones, who on paper has the best record also has a few controversial decisions (Gus, Santos, Reyes), which make people less certain of things.
Skill: The actual skillset of the fighter. Who was more and better rounded? Who excelled more in actual MMA skills. DJ and GSP are examples of superlative fighters that were extremely well rounded, on top of being dominant and having great records. Someone like Silva or Khabib, while not as well rounded, had superlative skills in a few areas that compensated for weaknesses in others. Someone like Cain was extremely skilled in most places, but didn't have the record to match it up.
A fourth, crucial but controversial one:
PEDs/cheating: Did the fighter have proven PED use in their career? If one cannot assume other fighters were in fact cheating without substantive proof, how does this affect? Some people will say cheating once discredits one's entire record or eligibility for being considered. I am in that camp. Others will claim it only disqualifies consideration on those wins that a person was flagged for. Yet another, more lenient one, will claim that since it is likely everyone or most were cheating, PED flagging is irrelevant.
Yet a fifth, related to record, is more finnicky still:
Incompetence/Competence in Judging: If a fighter has a loss on their record, or multiple losses, as a result of incompetence in judging, corruption, or sheer loopholing, should they be penalized? Should the one unduly rewarded be rewarded? Should terrible decisions be amended? Should we take Diego's loss against Pearson against him? How much does Yan's loss to Aljo via DQ detract from his record?