Most soulful female vocalist

Aretha Franklin, she's the Queen of Soul.

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?

Aretha

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.