Most soulful female vocalist

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.