Janis is a great pick... though I didn't think of her in the moment I made mine. Might have picked her, too, if I had.
A lot turns on what you consider to be rock, because if you are picky about genre, a lot of great female singers may not be considered rockers. For instance, how about Linda Ronstadt? In latter days she has crossed a lot of genre boundaries, but she made some rocking records back when. Would she still qualify? Or Aretha Franklin--a great, but a rocker?
Then there's Grace Slick. Definitely worth remembering as a performer, personality, creative force and icon. But was she a great singer, really?
And Bonnie Raitt could well be nominated. But there's the genre thing again--while lots of her songs are considered rock, she is so identified with the blues.
All in all, and considering only mainstream figures, the Wilson sisters of Heart would probably get my vote--great singers both. Nobody had soul power like Janis did, but as pure vocalists the Wilson sisters were better.
But if obscure is cool, I'd definitely go with Toni Garthwaite, of Joy of Cooking. She was a rock pioneer, a dominating and creative singer, a terrific writer, a huge personality--and since she's been performing since 1967, she's a true survivor.