There are two female voices that always do it for me:
Kate Bush -- her first two albums, not so much. But from the fourth album onwards, that voice is heavenly. No one can touch Kate when it comes to absolute beauty (voice-wise).
Shirley Manson -- the first time I heard a song from Garbage, I froze in my tracks. Who is that? Shirley's voice is the most sultry, aggressively sensual, sexual voice I've ever heard. She's got a voice that is, to be blunt, like sex. her voice just squirms into my brain and keeps caressing and stroking my senses until I can't take it anymore. Take that however you want!
cheers,
supervij