Fantasy author J.R.R. Tolkein (Lord of the Rings) is among the literary heavyweights said to be particularly enamored of the words cellar door. The combination has often (and apocryphally) been called the most beautiful words in the English language; cited for their melodic quality and the way the musical sounds contrast with the ordinariness of the object they describe.
I happen to believe that Dejah Thoris is the most beautiful name for an SF/fantasy character ever. I remember hearing it and instantly being enamored long before I ever read the source material. Dejah Thoris is, of course, the title character in Edgar Rice Burroughs’ seminal novel A Princess of Mars, which introduced the Earth to swashbuckling hero John Carter — who would go on to become the Warlord of Mars.