The On Language column in the New York Times magazine last Sunday was devoted to the most beautiful words in English, and it focused on the surprising frequency with which "cellar door" gets cited.
Cellar door has a mellifluous ring, but as an exquisite phrase it's like the beautiful girl with nothing upstairs. All it has going for it is sound. Summer afternoon - which Henry James claimed were the most beautiful words in the English language - is better, for its mixture of pleasing sounds (at least with the first word) and pleasant evocations.
When James Lipton asks an actor for his or her favorite word, the answer is often based entirely on meaning (making it the opposite of cellar door). "Tolerance," he'll hear. Or "Harmony." Interesting that no one has ever said "Oscar."
My favorite word has both a beautiful sound (at least to me) and a candlelit association. When I first heard it as a child I was enchanted (I still am) by the magical way that it takes one word and turns it seamlessly into another: evensong.