Aaagh too many. Here's only a "small" few I can remember for now sweatdrop Let's see, there's Ella Fitzgerald, Billie Holiday, Lena Horne, Astrud Gilberto, Dionne Warwick, Patrice Rushen, and Nancy Sinatra.
For newer-ish ones, there's Bic Runga, Les Nubians, Mikami Chisako from fra foa, Bebel Gilberto, Lea Salonga, Mary J. Blige, Juju, Ximena Sarinana, Lisa from m-flo, MISIA...
... and Whitney Houston before all that crack made her whack I mean Bobby Brown. crying
I would have to saaaaaaay:
Sumi Jo, Billie Holiday, Kristin Chenoweth, Barbra Streisand, Florence Welch, Maria Callas, Whitney Houston, Karen Carpenter, Mama Cass, Amy Winehouse (I've honestly been a huge fan of hers before her untimely death), Sarah McLachlan, Sara Bareilles, Edith Piaf, K. D. Lang, Bjork, Naya Rivera (The girl who plays Santana on "Glee." For some reason, I can't get enough of her voice.), Patsy Cline, Martina McBride (I'm not even a fan of country music, but she is a wonderful singer!), Christine Ebersole, Kerry Ellis, and Sutton Foster.
I wanted to see if I could come up with 20 names to put on here, and I'm glad it wasn't too tough for me! :D
Beth Gibbons of Portishead is my favourite female singer by a wiiiiiiiide margin.
I'm also a big fan of Bjork, Origa, Florence Welch (a lot with her recent popularity), and Fiona Apple. Then of course we have classics like Stevie Nicks, Grace Slick, and Janis Joplin. Dolly Parton earns an honorable mention as well.