What are your top 3 favorite female singers?

Sandy Denny has been mentioned.  Because she was chosen as the only person to sing duet with Led Zeppelin, I'm gunna have to dig into any possible recordings.

On theme, Alison Kruass who has done a couple things with Plant in recent years.

Merry Clayton who features on the Stones Gimme Shelter.

From Wiki,

Summoned from bed around midnight by Nitzsche, Clayton – about four months pregnant – made her recording with just a few takes and then returned home to bed. It remains the most prominent contribution to a Rolling Stones track by a female vocalist

* Aimee Mann

* Sarah McLachlan

* Ann Wilson

Joni Mitchell - "Blue"
Diana Krall - anything
Susana Baca - "Eco de Sombras"
Rosemary Clooney - "Do You Miss New York"

Ok, that's four.  But then I like Sade a lot too... 

I don't think anyone is overlooking anyone. The OP says 3. In another thread, I listed many many more.