exceptional voices of female singers (non-classical)

What I'm talking about here is what critics of classical singing refer to as "the instrument": the exceptional beauty or uniqueness of the timbre of the voice qua voice.  For the moment, put aside preferences w.r.t. repertoire, interpretation, performance, style, delivery, etc., and just consider the *sound* of the voice.  We're *not* talking about favorite female artists here, because that kind of preference is based on a whole amalgam of factors.

I suspect some of the top figures in the category I've tried to define often aren't very popular among audiophiles, because of those many other factors.

My first three nominations: Barbra Streisand, Linda Ronstadt, Eva Cassidy.