And Elsa

Merida was kind of like the villain of her story. She made the same bad decision as Mordu did. She also has a temper and doesn't think before acting, think of possible consequences, or how her actions may impact others.

She lost her temper and hit her own sibling in the chest with ice, which pretty much killed Anna. Definitely Elsa.

Merida, also. You want that badly to change your mother's mind that you will resort to a spell that changes her bodily, and then, after that, you may lose her forever?

Small wonder, that these princesses rate low on my favorites list.