Imma just leave the numbers out there and you can make your ranking.
-Valentina Shevchenko (FLYW): 7 title defenses in a row, 4 finishes
-Ronda Rousey (BW): 6 title defenses in a row, 6 finishes
-Amanda Nunes (BW): 5 title defenses in a row, 6 title defenses overall, 3 finishes
-Joanna Jędrzejczyk (SW): 5 title defenses in a row, 1 finish
-Rose Namajunas (SW): 2 title defenses overall
-Zhang Weili (SW): 1 title defense
Julianna Peña, Jessica Andrade, Carla Esparza, Holly Holm, Nicco Montano, Miesha Tate and Alexa Grasso are girls that have got the title but have no title defenses (yet).
I'm not even going to count the FW division since that one has always been a joke (can't even make a top 5 out of it).
So the one with the most title defenses is Valentina (7), the one with the biggest finishing rate is Ronda (100%) and the one who has fought the toughest competition idk because that one is subjective but I usually hear ppl putting Amanda in that regard