Lou Lou Ghelichkhani, Natalia Clavier, Shana Halligan, Elin Melagarejo. All of these singers perform on many of the Thievery Corporation releases. Thievery somehow finds the most beautiful and angelic female voices from around the world. Check out the Thievery Corporation release "Saudade". This album features all these female artist and more in a mordern Bosa Nova style...excellent!
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.