Here are 3 who haven't been mentioned, which I find hard to believe as this (great) thread is now running for five years!
Kiri Tekanawa doing any Gershwin, Cole Porter, or the role of Maria in West Side Story...to me the most exquisite female voice ever.
Elaine Paige is a Brit who has spent most of her career doing musicals. Her CD "Encore: Elaine Paige" is a wonderful introduction to her talent and includes such as "Memory" from "Cats," Piaf's "Hymne A L'Amour," and "Don't Cry For Me, Argentina" done the best I've ever heard it done.
Gladys Knight has a great voice but her style is what sets her apart for me. This gal knows how to sell a song! I would suggest starting with "The Best of Gladys Knight and the Pips."
This has been a wonderful thread for me as, through it, I have found my way to Cassandra Wilson, Patricia Barber, Holly Cole, and more of Jennifer Warnes beyond "Famous Blue Raincoat." (others, too)