They're both relatively sensible and level-headed, but have a tendency to not look on the brighter side of things.

And Elsa

Either her or Elsa. I feel like Aurora always had a touch of melancholy to her, because she was "lonely." I respected that. Also Elsa, because she feels like she has to push people away to protect them from herself.

agree with ace2000. or Tiana, being a pessimist imo.

Mulan is not pessimistic, but I can see her Sadness is like Riley's mother. Just like Sadness's role in the movie, Mulan went through all of the journey is for her family. Or Pocahontas and Aurora. I don't think really Elsa fits being Sadness because if so, she should have return to her family and told her all problems in tears, as Riley did. I always open my feelings when I cry.