The Princess Bridle
You say that like it's a bad thing...
I can't disagree with voting for Princess Celestia, because she really does have an incomparable majesty all her own. And it's not just her looks, but her bearing, personality and strength. She never has an unkind word for anypony (except villains who threaten her subjects), and leads by example. Her psychedelic pastel rainbow hair is also somewhat hypnotic and punctuates the fact that she is truly like no other.
I also can't disagree with those who voted for Luna. Her physical appearance, with her beautiful coat, moon symbols and flowing star-field hair are the perfect counterpart to Celestia's radiance. As the younger sister and Princess of the Night, her motif is dark and mysterious, and there is much we don't know about her. Despite her naive, youthful personality, her backstory as Nightmare Moon, her thousand-year banishment and her tendency to punctuate her sentences with thunderbolts make it clear that she is every bit Celestia's equal, and a powerful elemental force in her own right.
When it comes to Cadance, I can't help feeling these episodes could be renamed Mary Sue Got Married -- to Marty Stu, the perfect husband. She's so perfect, in fact, that I'll admit preferring the Chrysalis version at times, just for the depth of character.
Asked as we are who the most beautiful is, however, I had to vote for her. The regal sisters have a lofty, unearthly beauty, but Cadance has a youthful glow and colors that stand out in a field of standouts. Where Celestia and Luna are more like noble queens than princesses, Cadance is a true princess in every sense.
Her unpretentious, loving, down-to-earth personality is icing on the cake, but very important icing and part of the complete package. Queen Chrysalis gave us a chance to see what she would be like if she was a haughty, insensitive bitch, and in that form, she lacked almost all of the appeal of the real Cadance, which shows that her beauty isn't just hide-deep.
Cadance is the princess who cares about everypony, and holds herself above nopony. Her royalty needs no herald nor crown. She's a natural, it's an inseparable part of who she is.
Where Celestia is the Princess of the Day, and Luna is the Princess of the Night, Cadance is the Princess of Love. It is her special power, unique among all ponies.
How could I not vote for her?