Who was the smartest between

I’d have to go with Nellie here. She seemed to have more prairie smarts (The 19th century equivalent of street smarts) and was also somewhat academic. For a smart girl, Mary could be pretty dumb at times.

By the way, glad to see that you made your way here Anna. I remember you from IMDB 🙂

Nellie, she was conniving. I don't remember Mary ever conniving at anything, but I am not a superfan and don't remember the show that well.