BEST FEMALE VOCAL EVER

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

All of the ladies mentioned here are great. It would be hard, if not impossible, to rate just one as the best. A lot depends on personal music tastes and styles.
Each have performed fantastic songs while other of their songs were not so memorable. Versatility is certainly a large factor and requires a lot of talent. Maybe we listen to the tonality and depth of their voices even performed in songs we don't particularly like.
To all the ladies mentioned here.....not just one is best. You are all GREAT.