Here are my favorite singers of jazz, in no particular order of preference. I can't rank them really. Did I do this before? If I did I apologize:

Dinah Washington. She's like Billie Holiday but with a stronger voice. Very bluesy even in her more popular facets. Check out "Dinah Jams."

Ray Charles. I know he isn't a "jazz singer" per se. It doesn't really matter to me. Make your own list if you don't like it,

Billie Holiday. That's pretty obvious. She is simply one of the great jazz musicians period.

Armstrong. Yes, I know he can be quite awful. I can't stand to hear "Hello Dolly" or "Mack the Knife." You have to go back to the early days to find better stuff by him. How about the duets with Ella? You can't beat those.

Nat King Cole. Ok. That's an interesting one. He is obviously a jazz musician, but is he a jazz singer? Or does his singing represent a turn away from jazz? I love his voice and his phrasing. He inspired the early Ray Charles, even. He is "jazz" even when he is not "jazz."

Ella Fitzgerald. Another obvious one. Get away from the novelty numbers and into the song books.

Billy Eckstine. The male Sarah Vaughn? You got to give him his due.

Sarah Vaughn. I love that over-the-top quality to her performances. There is the sheer power and range of her voice, but I prefer her when she is the most "swinging." I hated her "Send in the Clowns," maybe because I hate all of Sondheim's music.

Sinatra. Ok. Sinatra could be awful too, when he became his mannerisms. A lot of singers do that. I like him best with those Nelson Riddle string arrangements. Sweet.

Antonio Benedetti. Tony Bennett is the purest jazz singer among the Italian crooners. He has his mannerisms too. If you like those mannerisms you will like him.

Shirley Horn. Not for nothing was she Miles Davis' favorite singer.

Bessie Smith. This was jazz singing before there was jazz. It's the original source--or one of them at least.

This is a tricky category because it merges into plain old "pop" singing at a certain point--at one end--and into rhythm and blues at the other end. It's interesting that there are more pure jazz singers among the women, and more cross-overs among the men. Ella, Sarah, and Dinah are "jazzier" in the generic sense than Frank, Ray Chas., the later Armstrong, or Nat Cole--or even Mel Tormé, who doesn't make my list.