Billie Holiday. Easy, hands down.

But other top choices are:
Janis Joplin
Grace Slick of Jefferson Airplane
Chrissie Hynde
Bonnie Raitt (and can she paly a mean guitar!)
Ella Fitzgerald

Nelly Furtado, Lucinda Williams, Stevie Nicks, and Shakira for their unique vocal styles

And for the modern pop-crowd:
Cristina Aguilera for sheer vocal control and power
Fiona Apple, because her delivery makes the little hairs on the back of my neck stand up.