Beth Ditto ....... of Gossip
Beth Hart
Nanci Griffith
Patty Griffin
Patti Labelle
Brittany Howard ...of Alabama Shakes
Lisa Hannigan
Julia Stone
Tanita Tikaram
Marketa Irglova
Gladys Knight
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.