I saw Heart live in Tempe, Arizona around 1978. Enjoyed the show, from what I can remember.
This thread prompted me to listen to some Patti Smith. Surely top ten, maybe top five, for female rock singers of that time, along with Stevie Nicks and "Blondie". Obviously there's no right or wrong answers here, except I'm taking Grace Slick with Tina Turner a very close second.

If I move into the 21st century, I'm taking Beth Hart, whose voice can be rocking a la Janis or weighty/bluesy... her cover of Etta James's
I'd Rather Go Blind with Joe Bonamassa stirs my soul every time I play it.
Much of this is genre dependent. If I veer outside of rock (which for the most part has been the focus of the OP and ensuing posts), and if I am just opining on the greatest female singer, it's not even close. Whitney and Aretha, 1 and 1A. I'd also be remiss if I did not mention one of my personal favorites (even if she isn't considered in the Top XX lists): Roberta Flack.