Sarah

 I was listening to a song by Sarah Vaughn in the car. The voice is of incredible richness, of course, but she also uses a variety of timbres and "tones of voice," whether serious, sassy, whimsical. The low notes do not have the same timbre as the high or medium registers. The vibrato is very interesting, because the speed of the vibrato varies: a long note the starts with no vibrato, then a vibrato that accelerates, then disappears at the very end. Normally, singers want a consistent timbre without breaks between registers, but not her. Also, with the vowel sounds, typically singers will alter them to create a more cohesive effect, but she goes all out and makes them sound different from one another. (The distinctive quality of a vowel is its timbre, after all. That's what makes one vowel different from another in the first place.) 

{It is odd, but with some songs that are not very good songs, she is unable to make them sound good. She can sound strangely awful and in bad taste especially if the arrangement is corny too.} 

Variations in intonation are very deliberate, as are melodic alterations. She also could play piano and knew the harmonies. With some other jazz singers, like Abbey Lincoln, I've always felt that the variations of phrasing and intonation were just ... out of synch and out of tune. I know Abbey Lincoln is an icon for many reasons, but it just not the same thing as my top female jazz singers, in no particular order, and hardly very original:  

Sarah Vaughn

Ella Fitzgerald

Billie Holiday 

Dinah Washington