Most soulful female vocalist

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.