Honestly, Merida, once older, is going to make a great ruler. She has conviction.

Next would probably be Tiana.

Either her, Poca, or Tiana.

Either her, or Merida. Maybe Mulan too.

Or Mulan. Merida not at all.

She has confidence, think quickly on her feet and a great fighter. She can rule with conviction and courage. She can unite people and fight for the right thing. But only after having learned it by the end of the movie.
Mulan and Poca are too indecisive until pushed to the edge of taking an action, which is a bad quality for leader.

Not Merida. I don't even think that she changed that much at the end of the movie. She changed but I still refuse to consider her as hero.

Willing to give up your dream to rule a people you've never met? Responsible? Kind? Gentelel? Supernatural charisma (handy)? Humble and knows what the common folk go through? I agree with RhythmicMagic in her analysis- she has the makings of a good queen. Pocahontas would also be good if she were more interested in the day to day kind of ruling like Moana does, which I like to think she becomes more serious about after the end of the movie. Mulan if she had an interest and Cinderella too- both modest and wouldn't let the pampered lifestyle get to them.

Merida doesn't seem interested at all in ruling and needs to learn to be less impulsive and before she should try her hand at ruling, imo. Making rash decisions based on how angry/upset you are is asking for political trouble.

@AudreyFreak, I agree that she would make a good queen but I don't see her as leader. I see her rather as a good advisor to Philip.

Or Mulan. Pocahontas would be a great chief.