Willy, you didn't include Holly Blue in your list....:biggrin1:
DC_Deep ... Are you trying to get me into trouble?
Holly Blue's vocalizations are uniquely extraterrestrial sounds. It would be patently unfair to have my other favourites compete with that!! 
Here's my thoughts on my favourites:
Dinah Washington: I've never heard another singer so aggressively attack a note, yet unneringly do that in a musical way. Her singing turns me on: no matter what she's singing, she oozes sexual confidence.
Keely Smith: Effortlessly sultry and smoky. Her live recordings from her early twenties, with Louis Prima are amazing. A very underrated singer.
Ella Fitzgerald: She could sing the phonebook and hold your interest. I'm not a big fan of her earliest and later recordings. But singers of the great standards will be forever indebted to her for her "Songbook recordings", done at the height of her vocal mastery.
Peggy Lee: She was the antithesis of the "Banshee Quartet"
decried by DC Deep in his post below. She had quite a small voice, but got your rapt attention nonetheless. More than almost any other singer, she really knew what dynamics were all about. I love her definitive version of Fever, but it's her late 1940's recordings with Benny Goodman that I'd take to a desert island.
Jo Stafford: Not really a jazz singer, but with the right material, she could melt your heart with the simple beauty of her voice and the honest emotion she poured out.
Yes, the singers mentioned above are all either dead or considerably older than me. I do listen to newer singers, but none that have come along in the past 25 years move me in the ways that my favourites do. I think it's much more difficult to become a truly entertaining,
musical singer today than it was say sixty years ago. The great singers of that era learned at the feet of skilled arrangers and virtuoso instrumental soloists in the topnotch jazz groups and Big Bands. Today's axis of "Over the Top" at one end and the somewhat isolated singer-songwriter at the opposite pole leaves an empty void. I think it's much more difficult to blossom as a great singer in today's totally different environment, and my hat is off to those who are making a valid contribution.
The one "newer" singer that I consistently enjoy is kd laing. She's got a fabulous voice and she's able to dip almost anywhere into the pop/jazz musical canon and not sound overly derivative.
