No one's mentioned Beth Gibbons of Portishead yet?! Her voice is amazing...absolutely haunting on many of the tracks. The live version of "Sour Times" is to this day the only song that's ever made me cry she sings it with SO much emotion.

I also like aretha franklin, just power and soul.
I had an ex who was a big Fiona Apple fan and I grew to like her voice a lot too.