You're right. Because clearly the list of amazing female vocalists is as endless as there are opinions. Since the OP was specific in asking for best female ROCK vocalist, I couldn't help but to go back to the time when what we now know as 'classic rock' was all the rage. When I think about the power female rockers of that era, most of the women discussed in this thread came to mind. But the later these great singers came along, the less I personally know about their discography. But I do know the music of Joplin, Benatar, Jett, etc. And while not in the classic rock category, there were no purer voices than Carpenter, Ronstadt, Houston, and a small handful of others.
While I'm sure that Lady Gaga, Miley Cyrus, and Pink can belt out rock tunes with the best of them, they simply weren't the ladies of 'my time'. Thus it's hard for me to associate their great voices into the category that was being specified by the OP.
This reminds me of the interpretation I give to the classic song by Don McLean, American Pie. The line: "The day the music died" has always meant (to me) when the music that appealed to us personally is no longer part of the mainstream. By extension it could have a much broader meaning to include many of the things from our past that were the staples of our daily lives but which are no longer popular or possible. When there are things that are such an important part of your life that you can look into the future and imagine a time when you will no longer be able to experience these joys, it might be easy for a person to conclude; "This'll be the day that I die".
I can't honestly say that I was ever a connoisseur of music as you and many other members here are. But I did attach myself to the music of my youth and I tend to compare what I hear now to the genre that 'spoke' to me in those formidable years. Sadly, because of that narrow method of critiquing music, I can hardly tolerate most of the styles of popular music today. However, as a former vocalist I can always appreciate great vocal skills; no matter the genre or the gender.
I will always be attached to the style that I grew up with. To me that is what music is. It's my inner definition of what music is supposed to be. When I hear new sounds & styles, I recognize that this is the very definition of art. If the styles of music doesn't change, it's all just covers of the past. For many of us older folks, the nostalgic side of music is far more attractive than the art side of music. There very well may be many more Ann Wilson-quality & style singing voices out there. But for now, she is my standard in this category. And I'm probably not going to be spending a lot of time looking elsewhere for another one.