I don't think the weakest champion is most likely to lose their belt.
As far as skill wise, I would say O'Malley and Poatan are the weakest of the bunch, both having obvious gaps in their skillsets.
However I don't think either will lose their belts any time soon. UFC are building O'Malley and seem content to give him stylisticly forgiving fights for him for the time being, same with Poatan. We may see him go for a 3rd belt before defending his own. Ankalaev could be a horrible matchup for him but he has the fight IQ of a twig.
I actually think Grasso might be most likely to lose the belt first given how strong her upcoming competition is.
The winner of Fiorot/Blanchfield will be a hell of a test for her, assuming she gets past Valentina again anyway.