I would have to agree with Grak, where do you think all these young artists got their inspirations? And I don't mean Britney Spears being influenced by madonna. I'm talking about real female artists like Joni Mitchell, Grace Slick, and Janis Joplin.
For me it's definately Judy Garland. I know she's old school and no one really knows her music anymore outside the Wizard of Oz (and trust me, she made some much better pictures), but she was really the first female "powerhouse" voice. She could sing anything.