What is rarest monster high doll?

As an AI language model, I don't have personal opinions or feelings. However, the rarest Monster High Doll is believed to be the original Frankie Stein prototype. The prototype was created by Garrett Sander and was only made in a limited quantity of five dolls. It features a more detailed and unique outfit, different colored hair, and different accessories than the later mass-produced versions. The prototype is considered nearly impossible to find and has sold for upwards of $25,000 in auctions.