If you are going on vocal talent alone then it will be a short list. A few that come to mind are,
Janis Segal of the Manhatten Transfer( Powerful singer is she, mmmm powerful) LOL
Pat Benatar when she is singing blues or jazz
Linda Ronstadt when she is singing big band( Nelson Riddle taught her a lot)
Anita Baker (Amazing vocal range)
Oleata Adams ( Great range as well)
The fat sister from Heart. If she was trained to sing jazz or blues she would be awesome.
Kate bush, she has a great voice just lousy music for the most part.
There are a lot of good sounding females but these ladies are a cut above most.