Most unusual/mesmerizing female voice.

I have seen Natalie Merchant several times, in different venues. When she starts the songs from "Ophelia", it can only be described as mesmerizing. The audiences seem to agree.

Eva Cassidy - who else can give you pause just by singing oooh?  One of a select group of performers recognized by only her first name.

Eva Cassidy - Fields Of Gold
https://www.youtube.com/watch?v=9UVjjcOUJLE

Eva Cassidy: Timeless Voice
https://www.youtube.com/watch?v=aDN_ss1S_5Y&t=4s

I second Frazey Ford and add Minnie Ripperton, Natalie Merchant and Janis Joplin

Eddie Reader
Cerys Matthews
Kirsty McColl
Georgia Brown
Lottie Lenya

Macy Gray