“Beautiful” here means: pure, free of distortion, an unalloyed pleasure to listen to. Like a perfectly tuned musical instrument. A strawberries-and-cream voice.
(Not the same as “most expressive voice”, an accolade which might well go to Maria Callas.)
There is an excellent case for Kathleen Battle; particularly when singing Mozart, which brings out her strengths.
Battle made a superb recording of Mozart arias with André Previn in the mid-eighties. You can listen to one of the tracks here.