She is kinda like Fedor - had a very long undefeated streak, mostly in Japan, against comparably weaker opposition. I do think she is probably the WMMA GOAT but there is a good argument for either Joanna or Ronda too.
Cyborg is in the Megumi category but is even more untested, with only a few other wins against true competition (Gina Carano, mainly...MAYBE Shayna Baszler although it was a looong time ago, arguably Coenen too but such a huge weight discrepancy there). Her most recent win against Holly was extremely impressive though - a few more wins against high level competition like that and she would be in the running.