Most unusual/mesmerizing female voice.

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