I think it is a little silly to talk about "most beautiful voice," because people have different ears. But here are a few candidates for pure beauty of sound; I'll put the sopranos first.
- Leontyne Price. For my money, the great LP had the most beautiful soprano voice evah, though there's plenty of competition.
- Rosa Ponselle
- Edith Mason. If you've never heard of her, it's probably because her career was mostly conducted in Chicago, where she was greatly loved and where her six marriages, two of them to the same husband, must have been an extremely entertaining scandal. You can find her recordings on an out-of-print Romophone CD.
- Montserrat Caballe
- Zinka Milanov
- Ebe Stignani
- Margarete Arndt-Ober
- Jussi Bjoerling
- Beniamino Gigli. Ignore the sobbing and other stylistic nasties, and avoid that recording of "Il mio tesoro."
- Giuseppe DiStefano, before he decided that a great Alfredo and Edgardo should be singing roles like Calaf and Manrico
- Fritz Wunderlich
- Robert Merrill
- Joseph Schwartz. You've probably never heard of him either. German, among the great Verdi baritones of the 20th c., died young of alcoholism.
- Ezio Pinza
- Alexander Kipnis
Your candidates? Yeah, Swenson, probably the most beautiful voice I've ever heard live. I probably ought to add Frida Leider and Christine Brewer to the list above.