Most soulful female vocalist

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