If there was to be a romance story about a frog and pony falling in love, particularly to serve as the pony version of stories like “The Frog Prince”, which of these mares do you think would be perfect for serving as the love interest of the frog?

Applejack because she would think it's weird at first

I'm not sure about "the best", it seems a weird premise in general. But I can see with Daring Do it starts out as her being helped out somehow by the frog, first wanting nothing to do with him because she works alone. Then reluctantly/begrudgingly starting to accept his help. Then starting to rely on it and appreciate it, forming a strong band as adventuring partners/friends. Eventually evolving into romance and breaking of the curse (if that is the end goal).