“cellardoor”

Onomatopoeia is a fiction. I discovered its illusory character when many years ago a Japanese friend with whom I often discussed literature told me that to him and some of his English-speaking friends the most beautiful word in our language was “cellardoor.” It was not beautiful to me and I wondered where its evocative power lay for the Japanese. Was it because they find l and r difficult to pronounce, and the word thus acquires remoteness and enchantment? I asked, and learned also that Tatsuo Sakuma, my friend, had never seen an American cellar door, either inside a house or outside — the usual two flaps on a sloping ledge. No doubt that lack of visual familiarity added to the word’s appeal. I concluded that its charmlessness to speakers of English lay simply in its meaning. It has the l and r sounds and d and long o dear to the analysts of verse music, but it is prosaic. Compare it with “celandine,” where the image of the flower at once makes the sound lovely.
— Jacques Barzun, An Essay on French Verse: For Readers of English Poetry, 1991, 51.

One cannot completely dissociate meaning and sounds. A Japanese said that the most beautiful word in English was “cellardoor.” This had not occurred to me, and never would have, but as soon as I heard this I remembered that “celadon,” which is so much like it, had always seemed to me a word of unusual beauty. Shelley and shelly do not sound alike, nor do Abraham the Prophet and Abraham Lincoln. It is even difficult for me to believe that “Cromwell, fling away ambition” is the same name as the Cromwell that goes with Oliver.
— Leo Stein, Appreciation: Painting, Poetry & Prose, 1996 (1947).

Last year a poll in England disclosed that the most beautiful word in the English language was "Mother." A nice sentiment, but a silly answer. A far better response, so a story goes, was proffered a century ago by an Italian immigrant to these shores. When asked what he thought was the most beautiful word in the English language, he replied: "cellar door."
— Arthur Krystal, "Hello, Beautiful," Harper's Magazine, September 2005.

With the Barzun, note “Japanese friend” and “his English-speaking friends” as well as “English-speaking” and “speakers of English.”

See also Cellar door, Wikipedia.