[DennisR]

Stevie Nicks
Tina Turner
Aretha Franklin

[Vince Ferrer]

Stevie Nicks

Tina Turner

Aretha Franklin

That is my list too. It all depends on what someone considers "best." I don't believe Stevie Nicks has a high quality voice, but more than any other singer I can think of, she makes the song interesting to listen to. She improves every song she sings.

On the other side, I'd rate one of the worst as Whitney Houston. She actually had a great voice, but didn't seem to know her limits. If she sang at an 8 or 9 level, she'd be one of the best. But she was often trying to take it to 11, and end up screetching rather than singing.