sexiest sounding female vocalists?

That's easy, Hope Sandavol from Mazzy Star!

Marcy Levy singing on "Lay down Sally"

1962 recording of Shirley Horn singing the now politically incorrect "Do It Again.' Off the Loads of Love LP. I don't ordinarily find Horn all that sexy, but damn. Otherwise, Helen Merrill in general and Diana Krall's "Peel Me a Grape" (though I still prefer Blossom Dearie's version).

Right now for me, it's Annie Haslam. She was the lead singer for the 70's band Renaissance.

2 great albums and great site:
http://rebekkabakken.universalmusic.at/