There are some reasons why I strongly think this way. If you don't agree with my opinion, please look carefully to my post; you might change your mind after reading mine.
First, Rapunzel has long pretty blond hair. If you are girl, you might thought of long pretty blond hair once. When I was young, I really wanted long blond hair like Rapunzel. It seems strange, but I think almost every girls want those pretty hair. Also girls think that long pretty blond hair is attractive.
Second, she is brave. To me, Brave means be able to try new adventure. Rapunzel was comfortable enough to live with her fake mother. However she wanted to see the lights which comes every time when her birthday comes. So she had and dangerous adventure. Actually if I was me, I would just be curious about the lights, not going out with a men. So I envy brave girls. They can at least try something and that seems so valuable things to me. Also it looks cool. How about you?
Moreover, she has magical power. I think not only girls, but also boys would be envy about the magical hair. If she covers the scar and spell the incantation, the scar gets better. She is so cool. How can she have those mysterious powers? To everybody, doesn't Rapunzel seems cool?
In conclusion, I will organize my opinion. Among the Disney princess including Merida, Rapunzel is the most positive role model for girls. She has all the things that girls envy and wants to get! Also she has happy ending!!! Oh how lucky she is!!!