exceptional voices of female singers (non-classical)

These are the only ones I've heard who belong in the top tier of "as close to vocally pure as possible".

Ann Wilson
I saw her in concert in her prime and when she sang Dream of the Archer it just sent chills through me. She lost it by the late 80's.

Oceans of Slumber's Cammi Gilbert
Check out their song Sunlight (Live) on Youtube, or their new song The Banished Heart
(caution: Metal, but I think most can handle it. It's mild.)

Bonnie Raitt
I don't know why her voice sounds so much better on ballads, but her rendition of I Can't Make You Love Me is stunning. Adele attempted this song and butchered it. Only Bonnie can pull it off.

Whitney Houston
Celine Dion
Not my style, but I appreciate their stunning talent.

@toolshedamps

Aziz Ali, Gillian Welch.......

Do you mean Azam Ali?  If so, +1

Thanks @bdp24 for recognition of Darlene Love. She never gets the recognition I think she deserves.
So many great nominees, and a few new to me.I'll have to do some research.
A few I didn't see:
Bonnie Dobson, who wrote and first recorded Morning Dew.
BONNIE RAITT, or did I miss her?
If you've never heard the all-girl group, ISIS; Carol MacDonald has a delicious contralto.
Speaking of contraltos, Chrissy Amphlett of the Divynals, Amy Winehouse, Johnette Napolitano of Concrete Blond..Luv 'em

Did we miss Margo Timmins of Cowboy Junkies?

There are too many amazing background singers to mention; but the Blu-Ray of 20 FEET FROM STARDOM covers many of them.
Of special merit are Clair Torey (Great Gig in the Sky) and Merry Clayton (Gimme Shelter). Rosemary Butler is another fav.

I don't know if we can count her, because she is an opera singer; but Olivia Safe contributed such amazing vocals to Jeff Beck's SERENE, that I thought it was a synthesizer. Never heard anyone hit the highs she does.

Three I am rooting for:
Nicole Atkins
https://www.youtube.com/watch?v=lefhIHdWs3I
Frazy Ford for her unique sound
https://www.youtube.com/watch?v=PXRrySTujn8
Bonnie Bishop, an evolving talent.

Did anyone mention Katie Malua ?

Karen Carpenter has the best female voice I've ever heard. 

I also like:
Christine (Perfect) McVie
Gloria Estefan
Carol King
Natalie Merchant
Clare Torry - "The Great Gig in the Sky" fame