Most underrated female recording artists..

Joan Jett.

Dominique Fils-Aimé - Nameless. Her voice is beautiful, and the album's recording is a reference quality. 

Emily King

@moonwatcher
I think it may be useful to provide a short list of artists that you have actually heard, not a list pulled from an algorithm that captures every vocalist in the known and parallel universes.

My list:

Karen Dalton
Sandy Denny
Laura Nyro
Eva Cassidy
M’bilia Bel - "The Cleopatra of Congolese music"
Zoila Augusta Emperatriz Chávarri del Castillo, artistically known as Yma Sumac
María Tintaya Rayo, artistically known as "La Calandria del Sur"
Tonia la Negra
Blanca Rosa Gil
Márta Sebestyén
Mirella Freni
Teresa Berganza

That’s a short list of people you may not have heard of.
I see others have already posted some of them.

@unreceivedogma I wasn’t. I was providing a consolidated listing of all those mentioned by everyone up until I posted to make it convenient for people to copy and paste into a Word document or an email to save for easy reference. Then I mentioned a few that I liked.

@moonwatcher 
Oh, cool. My bad!