All of them, but Peter Pan and Little Mermaid would be my top two

Peter Pan and Little Mermaid for sure.

Also the Frog Prince. Jack and the Beanstalk might be difficult to do, as would Goldilocks and the three bears. Peter Pan, while a cool character, falls in the same class as Alice in Wonderland does for me, that is it is a fairly modern day book, not a traditional folk or fairy tale so I don't think it really belongs on the show. And Midas is from Ancient Greece, I don't think he fits in with the rest of the show either. I also always liked the H.C. Andersen tale about the soldier and the magic tinder box and the 3 giant dogs.

Regina/Evil Queen's backstory; Emma's backstory; Sleeping Beauty; Wizard of Ozz; The Frog Prince; Goldilocks and the Three Bears; The Little Mermaid; Maleficent's backstory

I can't see this really working on the show but I had to pick it coz it was one of my fav Fairy Tales when I was a kid.

Peter Pan's a close second especially now that they've introduced Captain Hook into the mix.