Cellar Door
from the Wikipedia article:
The English compound cellar door plays a certain role in discussions of phonoaesthetics; a widely repeated claim first put forward by J. R. R. Tolkien in his essay English and Welsh (1955) holds that its sound is intrinsically beautiful.
Cellar door is a combination of words in the English language once characterized by J. R. R. Tolkien to have an especially beautiful sound. In his 1955 essay "English and Welsh", commenting on his affection towards the Welsh language, Tolkien wrote:
"Most English-speaking people...will admit that cellar door is 'beautiful', especially if dissociated from its sense (and from its spelling). More beautiful than, say, sky, and far more beautiful than beautiful. Well then, in Welsh for me cellar doors are extraordinarily frequent, and moving to the higher dimension, the words in which there is pleasure in the contemplation of the association of form and sense are abundant."