It's not fair to be asked to name one favorite...and anyway, the MOST beautiful ballads to me are the Scottish...

(A lot depends on tune....the same ballad can be hauntingly beautiful, stark and chilling, or just plain silly, depending on version and melody....various versions of The Three Ravens (Twa Corbies) can illustrate this) same with "Gypsy Davey" or "Matty Groves"

A long time favorite, (mostly because I know someone who sings it beautifully is "Ned of the Hill")