This is juvenile...

How about  Sara Bareilles from "Jesus Christ Superstar"? Such a beautiful Voice I don't even need to know what she looks like.  

Its all about the character of the voice, isn’t it? There cannot be one female vocalist fortunately:) Here is the list in no particular order:

The one and only - Tina Turner
Aretha Franklin
Whitney Houston
Edith Piaf
Janis Joplin
Stevie Nicks

My apologies to that person who likes Diana Krall. No offense to you but her voice is like very smooth and well churned vanilla. But its vanilla... esp amongst the great vocalists others have listed here on this thread. Diana Krall is like going and visiting stunning Venice on a massive cruise ship. Its as lacking in character as the fake Italian villas scattered all over Newport Beach coast in CA. There.

No-one mentioned her. She died very young. She is already very ill here and she knows it. Let's listen..
https://www.youtube.com/watch?v=2-Y_4tW0jnw

I second Sade. Classy woman and sexy as hell.

I third Sade plus 
Beth Orton
Melody Gardot
Carole King
Etta James 
Nina Simone
Sara Vaughn
Ella!!!
Beat that Stevie Nicks????
IMO