Others on this thread have commented on her pure voice. I, too, concur; but I would go so far as to put her voice on top of the world of unique female pop singers.
Others on my list would be:
Anne Murray
Aretha Franklin
Patsy Cline
Annie Lennox
Ruthie Henshall (known mostly to Brits)
Diana Ross
Shania Twain
Patti Paige
Janis Joplin
Also unique and memorable pop/rock singers:
Debbie Harry
Stevie Nicks
Christine McVie
Grace Slick
Joni Mitchell
Nancy Wilson
Kate Bush
You left off Allison Krause. Her voice is angelic.
You mentioned Janis Joplin. Above is a link to a song from Jefferson Airplane's first album with Signe Anderson singing “Chauffeur Blues”. Different style, but what a voice!
Patsy Cline for sure...Amy Winehouse although I never followed her... now I hear her and think ...what a great voice....
and there's also Eva Cassidy, Fields of Gold
most have never heard of her because she, too, died young