When I chose Sarah McLachlan in an earlier post I did so not purely because of her voice, but also for her incredible writing talent. Based on the thread title and selecting someone for vocals only I would have, and maybe should have, chosen Eva Cassidy. To me she is the female equivalent of Nat King Cole, who I consider to have possessed the greatest male voice of all time.
Hollly Cole for jazz, Freda Epstein for Folk, Best voice I feel belongs to the legendaryBarbara Streisand for what she can do with it. But for those who do not know Fredya and the Acoust Attitude on Red house records did an album called Midnight at Caball Hall and its some of the best acoustic folk I have heard and she tragically died at age 46 in a auto accident but the music is unreal.