Most prutty pone.
It is not only Raritys physical beauty that lets her rightfully claim that title, even though it alone would certainly suffice, but a combination of certain factors. For Rarity is not only beautiful in body, but also in spirit.
So great is the loveliness of the second that she is willing to endanger the first to do the right thing.
Risking her perfect body to be maimed by an angry manticore.
Letting an untested, mutating spell be cast upon her shapely form.
Exposing her silken coat to her biggest enemy: MUD.
And many more things.
This willingness for sacrifice that joins her physical qualities makes her the true, absolute beauty that she is. A shining, generous beacon of a soul in a flawless body.
Some say that true beauty comes from within, and is more important than physical beauty. This is true.
But Rarity has both! Watcha gon do 'bout it?