I'll stick my neck out on a couple of folks who have not yet been mentioned. Both are what I would call acquired tastes, one male, one female. The former would be Chet Baker, especially early to mid-50's Chet before he got crispy. Although, having said that, I'll admit that even late period Chet has its own peculiar allure, not at all unlike Billie's Lady in Satin.
The woman is Blossom Dearie. I find her to be truly unique. She has a fair number recordings available from her days with the Verve label. She also sang with a European vocal ensemble called The Blue Stars of France. Those CD's are much harder to come by. If you haven't ever heard her, she's worth checking out. No guarantees here, though. Like I said, an acquired taste.
If I was to dip myself in more populated waters, I'd have to go with Anita O'Day. I bought her Mosaic set a couple of years ago only because the price was too good to pass up. To say I was pleasantly surprised would be a major understatement. I've since become a big fan. She's a remarkable talent. I'd challenge anyone to find a jazz singer who exemplifies the term to a greater degree than Ms. O'Day.