Most soulful female vocalist

What female vocalist can sing soul like no other , and why .
LHasa De Sela. Only Soul of her left now, but I'm still enjoying her voice and songs. Ask Wiki for more info.
Aretha Franklin, she's the Queen of Soul.
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.
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".
To clarify, do you mean a female vocalist who sings in the genre usually referred to as "Soul," or one who sings soulfully?
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.
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.
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.
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 .
Bettye LaVette, Mavis Staples, Ruthie Foster
The Spanish singer Martirio, the Portuguese fado singer Cristina Branco and Lhasa De Sela.
Add Edit Piaf and Mirelle Mathiew -- France;
Ofra Haza -- Izrael(gone since 2/2000)... go Wiki
I would say the 'soulfulness' of a singer depends on the 'soulfulness' of the listener.
The properties of the emotional response of the thought, or feeling: "that is really soulful" is in the mind of the listener.
I have to agree some singers are more likely to be able tto bring out this emotional response than others, but it is a really personal thing.
So one persons exquisite experience of a singer eliciting a 'soulful' response, can leave another person with nothing.
Ditto for goosebumps, shivers, tears, raising your hackles..
Elizabeth, your so smart and insightful. A simple question with a simple answer.
FWIW, the first one that jumped to my mind was:
Billie Holiday.
It's Aretha Franklin,

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

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.
Janis Joplin had a hugh soulful voice.
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.
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.
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
Forgot about ... Holy Cole
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".
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."
Roberta Flack...charming voice.