In terms of ability Rosalina definitely takes the cake, and she's certainly the "coolest" of the lot, if her dignity and power is any indication. But there's something about Peach, some hidden psychopathic figure below her layers of pink frills and exaggerated helplessness... I don't know; I just wouldn't be surprised if she eventually went insane and killed anyone, or is secretly a super-spy, or is actually the creator of the universe, or something.
hmmm, perchance she is really the Goddess Hylia...