Most English speaking people, would admit that 'cellar door' is beautiful, especially if dissociated from it's sense (and spelling).
More beautiful than say, 'sky', and far more beautiful than 'beautiful'.
According to J.R.R. Tolkien, Cellar Door is a combination of words in the English language thought to have an especially beautiful sound. Not from meaning, so the words could be spelt in these ways, to eradicate meaning, enabling the mind to just hear them as sound: Celador, Selador.