Most underrated female recording artists..

One more add I forgot, Big Mama Thornton.  Old blues singer who gets lost in the mist of times.

If you like blues, Susan Tedeschi is a terrific singer.  You het the added benefit of Derek Trucks slide playing.  They did a live reprise of the entire Layla album that stands up to the original in every way, just amazing.

For a more traditional female singer I would offer Eva Cassidy.  She has a pure voice as demonstrated on Live at Blues Alley.  She was taken fromus far too young by cancer.