What is the most beautiful phrase in the English langauge?

Background: I was rewatching Donnie Darko several days ago. Those of you who have seen it know that the English teacher writes Cellar Door on the blackboard before she leaves. Donnie asks why, she explains that a specific linguist, after many years of study had described “Cellar Door” as the most beautiful phrase in the English language.

I always try to challenge the idea. It seems a subjective matter. However, I come up short. First I don’t understand the criteria, (simplicity, harmony, ... ?) and I am not ruling out the power of suggestion.

Does any one know what quality the phrase has that makes it appealing. Or have any other phrases they think are equal to or better then
Cellar Door