BEST FEMALE VOCAL EVER

To my ear there is no better voice than Karen Carpenter. When I hear her sing it just blows my mind. Not a fan of the music but her voice just kills me. A close second would be Alison Krauss. I could listen to her sing forever. I'll throw out a jazz singer who is not often mentioned but whose smokey delivery I enjoy--Carol Sloane. 

Never one best.
Nina Simone, Melody Gardot, Lady Gaga,
Amy Winehouse, Karen Carpenter, kd Lang (live).

Minnie Riperton. ‘Queen of the whistle register”. It’s a shame she died so young......

Judith Durham

+1 re Judith Durham.

Given the OP’s admonition to only mention one name, and based on my assumption that the thread was not intended to address opera and classical singers or to take repertoire into account, I cited Connie Francis earlier in the thread. However I could just as easily have named Judith, and in fact I pondered which of the two to cite.

Following are comments I made and quoted about Judith earlier this year in the following thread:

https://forum.audiogon.com/discussions/exceptional-voices-of-female-singers-non-classical

Based on the purity and sheer beauty of her voice, I would cite Judith Durham, best known as the lead singer of The Seekers although she has had a distinguished solo career as well.

Following is a brief excerpt of comments about her provided by a British gentleman whose bio is shown at the end of an article he wrote entitled Judith Durham: The Voice of the 20th Century. Obviously some of what he has to say in this paragraph and in the rest of the article is debatable, as well as being subjective to a considerable extent, but his comments are not without foundation IMO.

Judith Durham (of the Seekers) is the greatest of them all. The voice of the twentieth century, in my not very humble opinion. Clarity, spine-shivering (I’ve already used “spine-tingling” and I’m all for a bit of variety), astonishing purity – all that is obvious straightaway. But listen to The Carnival is Over or the amazing Just a Closer Walk and you hear a vocal beauty which is unique and reaches a level no one else can get near to. Callas is just a tear-jerker by comparison; Carpenter just a female crooner; Piaf just a screeching Gallic banshee. Judith Durham combines a bold, even brash, confidence in her own ability with a deeper humility which lends her remarkable sound an even more profound character.

Regards,
-- Al