Wow! That really can be a hard one to answer if you thought it through.
Freddy Mercury was awesome. Tina Turner had (Still does) some great tones out of that sweet mouth. Stevie Nicks has that haunting sound that you can't mistake but didn't ever really have a lot of highs to her songs. But Singer wise, there are TONS of gals in my opinion that can get those tones out there. Bionce, Fergie, Mariah and even dare I say them, Lady Gaga and Madonna have really done great in their pop songs. I don't think I can EVER put any ONE of them out there above the rest though. That's the beauty of music... it's what makes the listener happy, that makes the artists great. OH and if none of you have heard Lisa Mac sing, you BETTER go listen to her! She's another one that has one of those seductive voices as well.