Most soulful female vocalist

Roberta Flack...charming voice.

I don't know if there's such thing a "the MOST soulful singer," but Carmen McRae is way up there. Has a way of phrasing like no other. Check out "Carmen Sings Monk."

I put Billie Holiday's vocals in the same elite class as Miles Davis' horn playing. Different styles for sure but, to me, they are kindred souls using different tools in different ways. But that's not why I'm posting. I'm listening to "The Essential Nina Simone" as I'm writing, and the only word that comes to mind is "Soulful".

Forgot about ... Holy Cole

Hard to argue with the choice of Aretha, but soulfulness takes a lot of different forms, and like Elizabeth said, it's not always easy to know why an artist touches us. Many good choices above. Here's another. This woman slays me:

Elis Regina
http://www.youtube.com/watch?v=35FPZR24djg&feature=related

Lokie,yes, in the history of Soul, Aretha has no equal as the most soulful female vocalist. Those who don't know, don't know what they don't know.

Eslaudio: "four loko"?? Is that a liquid, solid or smoke?

Lithojoe: I thought I was the resident smart ass around here but evidently I have some competition.

Alright... lets get one thing straight, Aretha Franklin is the Queen of Soul. You can talk about nuances and emotional response all you want, but you are embarrassing yourself if you think anyone, man or woman, can sing more soulful than Aretha Franklin. So lets just talk about who's in second place.

Janis Joplin had a hugh soulful voice.

It's Aretha Franklin,

Don't believe me? Take a listen at this vintage live Youtube video of her singing "Dr Feelgood" from 1968

http://www.youtube.com/watch?v=7ExOYgjzLnU

By the way, Lady Gaga gets overlooked and ridiculed by the audio enthusiasts here, but she is really very talented and has some soul in her voice.

FWIW, the first one that jumped to my mind was:
Billie Holiday.

Elizabeth, your so smart and insightful. A simple question with a simple answer.

Add Edit Piaf and Mirelle Mathiew -- France;
Ofra Haza -- Izrael(gone since 2/2000)... go Wiki

The Spanish singer Martirio, the Portuguese fado singer Cristina Branco and Lhasa De Sela.

Bettye LaVette, Mavis Staples, Ruthie Foster

Sfar .... After rereading my question I can see the confusion . I meant the singer more than the genre .
A few responders mentioned Aretha , granted she was the queen of soul , but did her voice have that warm , stirring , willowy , soul sound . She could belt it out .
Surprised no one has mentioned Edda James .

Chaka Kahn.

She is a true singer who is very highly regarded and has an excellent track record since the mid seventies. Excellent voice and vocal control. Her vocals always display her connection and emotion with the accompanying musicians/music.

She has been featured on projects with several other well know artist.

Lady Gaga! After a long day at the widget factory, there's nothing like coming home, popping the tab on a Four Loko and letting the complex emotional nuances of her voice wash my cares away.

Nina Nastasia. So much emotion and sensitivity in her voice. She's an actual folk artist. Her albums are edited on the Touch and Go and Fat Cat Record labels, usually recorded by Steve Albini.

Aretha

To clarify, do you mean a female vocalist who sings in the genre usually referred to as "Soul," or one who sings soulfully?

Don't know about the "most", but the Afro/Spanish singer Concha Buika deploys a smoky, sandpaper voice suffused with flamenco-like passion. Check out "La Nina De Fuego" and her collaboration with the great Cuban pianist Chucho Valdes "El Ultimo Trago".

Joan Osbourne. Truly one of the most underappreciated female artists going. She has a voice that is one of the most beautiful, gale-force instruments I've ever heard in the pop idiom.

Aretha Franklin, she's the Queen of Soul.

LHasa De Sela. Only Soul of her left now, but I'm still enjoying her voice and songs. Ask Wiki for more info.