I don’t have ‘best female author of all time’ but I do have favorite writers some of which happen to be female. I don’t usually split them by their sex (nor by their height, distaste for bananas, or whatever) as for me they’re all in the same ‘people who have a great time staining paper with ink making me a happy reader’ league but here it is, in absolutely no order beside the first two, as there is them and then there is all the others:

• Virginia Woolf (the only reason I would love to be able to travel in time is to meet her),
• Jane Austen,
• Edit: (how could I forget) Emily Dickinson!
• Sylvia Plath,
• Shirley Jackson (if you have not already, go read The Haunting of Hill House, it’s considered a classic for reasons),
• la marquise de Sévigné (she wrote letters and they make for a great read, no idea if it’s available in English),
• Margaret Atwood (imho she deserves a Nobel Prize, next to Woolf and Austen),
• Mary Shelley (like mentioned by others already, she well deserves to be read and would still have a lot to teach to some contemporary authors too, imho).
• I love reading Lizza Tuttle. Her horror short stories are different.
• In the same vein, I also quite like Mélanie Fazzi (who is also a translator of some of Tuttle’s stories, btw). But I can’t find that much more female writers in that specific genre (a lot more males do come to my mind).

Being French, I realize I have not listed that many French female writers I would consider a favorite. But they are a few I would consider excellent read nonetheless:

• La comtesse de Ségur (one of my childhood companion next to, say, Verne and Doyle),
• Simone de Beauvoir,
• (very) few pages of Marguerite Duras,
• Fred Vargas.
• To which I would also add Pauline Réage, because I think her ‘Histoire d’O’ is well worth reading for anyone into erotica.
• At one time, I also quite liked Joëlle Wintrebert (scifi) but I have not felt like reading her for a very long time so I could not tell.