Its a fairly easy thing to assess on the women's side. Nunes has to be the goat of the women's game since she smashed both Rousey and Cybord and also beat Shevchenko twice, and to top it all off also beat Holly Holm, Meisha Tate, and DuRandamie as well.
Rousey and Cyborg would be next tier for their respective dominant stretches prior to facing Nunes.
Shevchenko, Joanna et al, would be tier 3.
I personally find Rousey like McGregor. Done a lot for the sport but when it comes to fighting ability, can't compare to Nunes/Khabib so wouldn't think they have much credit in the goat conversation.
Joanna is slightly different but Rose beat her twice and she lost to Zhang and Shevchenko so can't really say she's a goat.
Cyborg has a claim as she only lost to Nunes and if they run that back, it could be different. Not saying it will be, but the arguement can be made. But she mainly beat cans and did lose to Nunes so for me, Nunes is above her.
Shevchenko again only lost to Nunes but that's at a weight above her natural weight and the decision was razor sharp so there's a conversation there.
With that said, the female goats conversation (for me) is between Nunes, Cyborg and Shevchenko. I would order it:
Nunes
Shevchenko
Cyborg
If Zhang carries on the way she is though, she may enter that list...