cellar door.

"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." - J. R. R. Tolkien