If we're talking about bands fronted by women then I'm going to say Emily Haines of Metric and Karen O of Yeah Yeah Yeahs.

There are a bunch of female solo artists with voices that I love listening to; such as, Adele and Diana Krall.

joanna newsom, joanna newsom, joanna newsom!

Alizee has such a special voice, love listening to her music.
listen to the song La isla bonita

Looking at this thread made me realize I don't listen to many female singers. Wow.

I'd have to say Grace Slick is up there for me though.

although many (read everybody) disagree, Amy Lee of Evanescence <3

Grace Slick for me.

probably PJ Harvey with Karen O coming in just behind

http://greatsouthernbrainfart.files....e-sandoval.jpg
This lady right here. The sexiest and most engaging voice ever.

Ann Wilson or(If you count solo artists) Diana Krall. IMO, of course.

Allison Mosshart for her work with The Kills and The Dead Weather, obviously...