IMO, I don't think that most of the female jazz singers on today's scene hold a candle to the "greats" of the 1940's through 1970's, probably because the fine vocalists of that era learned their craft singing to live audiences, backed by stellar musicians in the big bands of the time. Among the "greats" I'd start with Sarah Vaughn (arguably the best woman jazz singer ever), plus Billie Holiday, Ella Fitzgerald, Dinah Washington, Ernestine Anderson, Sheila Jordan, Betty Carter, Nina Simone, Carmen McRae, Abbey Lincoln, and Shirley Horn. The list of excellent women jazz singers in the past 15 years would include: Diane Schuur, DeeDee Bridgewater, and Cassandra Wilson, with the following "new" names also deserving of mention: Kate McGarry, Nneena Freelon, Lorraine Feather, and Luciana Souza (chosen by recent Downbeat poll as top upcoming female singer).
If you really get interested in the history of female jazz singers, you should also go back in time and listen to the following blues/jazz artists of the 1920's and 30's who shaped the vocal art: Bessie Smith, Gertrude "Ma" Rainey, Bertha "Chippie" Hill, Ida Cox, Victoria Spivey, Sippie Wallace, Alberta Hunter, and Adelaide Hall.
If you really get interested in the history of female jazz singers, you should also go back in time and listen to the following blues/jazz artists of the 1920's and 30's who shaped the vocal art: Bessie Smith, Gertrude "Ma" Rainey, Bertha "Chippie" Hill, Ida Cox, Victoria Spivey, Sippie Wallace, Alberta Hunter, and Adelaide Hall.