The prior posts have covered a fair bit of territory. Ella Fitzgerald, in my opinion, is the quintessential female jazz vocalist. Sarah Vauhgan had a better instrument in some ways, but her delivery was a bit more quirky. The late Susannah McCorkle sang very well, although, for love or money, I can't understand why my wife always insists she was nasal. A singer whose instrument was not the mightiest but who made the most of it with a beautiful sense of delivery, great tone and a believable, I was there, I've done that, quality is Anita O'Day. Try "All the sad young men" on Verve.