Why not title the thread "Favorite female vocalist dead or alive?" There are way too many amazing unique female singers to think of one as "the best."
Plus, it might technically fall to an opera singer for that.
Gladys Knight, Linda Ronstadt, Aretha Franklin, Bonnie Raitt, Mary Chapin Carpenter, Joan Osbourne, Sade, Jennifer Warnes, Janis Joplin, Dionne Warwick, Stevie Nicks, and Grace Slick are some of my favorites... for different reasons.