#2. Kayla Harrison - 2-time PFL lightweight champion

Kayla Harrison is not even in the UFC, which is obviously a major obstacle. On top of that, it seems at the moment she is more likely to re-sign with the PFL. However, should she have a change of heart, she could one day end up as the UFC's female GOAT.
Harrison sports a perfect 12-0 record and pairing this with her two Olympic gold medals is quite the starting point. Should she join MMA's biggest organization, she would likely have to compete at featherweight, meaning any path to the belt would most likely go through Amanda Nunes.
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Beating Amanda Nunes, potentially multiple times, and maintaining a perfect record with a number of title defenses could make Harrison the female GOAT. While she might struggle for competition at featherweight, a perfect record after joining the biggest MMA promotion could see her be touted as the best female star the promotion has seen.
#1. Valentina Shevchenko - UFC flyweight champion

The current No.1 ranked pound-for-pound female is comfortably the most likely to overtake Amanda Nunes as the female GOAT. Valentina Shevchenko is in the midst of a flyweight title run that already includes six successful defenses and has boosted her record to 22-3.
The main issue with her current case are the two losses on her record to Amanda Nunes. While the second fight between the pair was highly controversial, she will likely need a win over 'The Lioness' to completely overtake her in people's minds.
Shevchenko can challenge the winner of the Peña-Nunes rematch. Winning that would earn her champ-champ status, which would be a big plus. From there, continuing to rack up title defenses and ideally beating Nunes at least at some point would make it tough to deny her the status of the UFC's female GOAT.