I don't know how to pick just one as the most beautiful. In the realms of classical and religious music there are hundreds of contenders. Even in the narrower field of Christmas music there are many beautiful melodies. I can't think of a more beautiful tune than "Gaudete". At the other end of the spectrum, and this may seem counterintuitive, I think Tom Waits has written some intensely beautiful songs. In spite of his ravished voice and reputation for gritty retrospectives on the seedier side of life, on repeated listening songs like "Innocent When You Dream", "Time" and "Jersey Girl" are just striking. Such a wide spectrum of beautiful songs is why I say it is impossible to chose just one.