The Good Bra.
It's spoken of in whispers, a relic so ancient and so powerful that most discussion regarding it is hushed. There is nothing more terrifying than imagining the sheer power of The Good Bra than the idea that it might not be real at all.
The Good Bra fits it's wearer like a glove, and will change their chest to the size they desire, functioning even as a binder for those who need one.
The Good Bra will never cut into you, it will never hurt your back, and when you run it will never hurt as long as you are wearing The Good Bra.
The Good Bra also functions as a Bra of Holding, because we all know the fabled Pants With Pockets are cursed and should never be worn.