Who are the best female authors of all time?

Astrid Lindgren needs to be mentioned.

Cornelia Funke is also really good.
Elisabeth Büchle is an author I personally really like.

Idk if best of all time, but I don’t see enough women in SciFi so they stand out when I stumble across them.

Ann Leslie, Octavia Butler, and Marta Wells come to mind.

Leslie is amazing IMO.

Suzanne Collins.

In my opinion, of course. The Hunger Games were shockingly well-written for YA at the time, and they ring truer by the day.

In the realm of high fantasy, Margaret Weiss doesn't get enough love IMO.

Her work with Tracy Hickman, both Dragonlance and Death Gate Cycle (especially the Death Gate Cycle) are some of my favorite books.

For sci-fi, I can't recommend Linda Nagata enough. The Nanotech Succession and Inverted Frontier are a couple of my all time favorite series.

Since Le Guin is covered I would like to spptlight Margaret Killjoy as well.

I haven’t read Earthsea, but I have heard nothing but great things about Ursala K. Le Guin.

I started Earthsea many years ago, not sure why I never returned to it. More recently I found The Dispossessed, and found it absolutely brilliant.
Also, "The Word For World Is Forest". It had a foreword, and I was thinking "okay, can we get to the story now?", but there was a second foreword. I sighed and almost skipped it, but it said "written by the author". And it was so perfectly written, and so interesting... I didn't know it was possible to write that well! The book itself was great too, but the foreword... okay I'll shut up now and go re-reading it.
Came here to say this and you beat me by seconds!
Ursula is great.

Depends on how you define "best". Most well known? Most read? Most highly awarded? Wrote the most? "Best" is such a vague and useless term here. I'll give it a go though.

Someone already mentioned Ursula K. Le Guin. So I'll add a few more along with their likely most well known book

Mary Shelly (Frankenstein)
Agatha Christie (Murder on the Orient Express)
Emily Brontë (Wuthering Heights)
Jane Austen (Pride and Prejudice)
Harper Lee (To kill a mockingbird)
Margaret Atwood (Handmaid's Tale)
Laura Ingals Wilder (Little House on the Prairie)
Beatrix Potter (The tale of Peter rabbit)
Ann Frank (her diary)
Margaret Mitchell (Gone with the Wind)
Anne McCaffrey (Dragonriders of Pern)
Octavia Butler (parable of the sower)

And just to spice it up a bit:

Hiromu Arakawa (Full metal alchemist)

I think that's a good start. I'll end it here or I'll be here all day

I think I'd put Virginia Woolf on my list.

Margaret Atwood should not be overlooked

The stand outs in my book(heh):

Agatha Christie
Mary Roach
Ursula K LeGuin
Madeline L'Engle
Mary Ruefle
Kathy Reichs

Hard to beat Naomi Novik in my book.

Diane Duane.

She's written Star Trek stories, the Young Wizard series, another series about a world of elemental magic, her writing is engaging and accessible and intelligent and tight, and she's honestly a really kind and nice caring person.

Loved Anne rice. The lives of the Mayfair witches was one of my favourite things when I was younger. The vampire lestat series was pretty great too.

Sincerly hope nobody comes in to tell me she's some kind of awful person

Sincerly hope nobody comes in to tell me she’s some kind of awful person
I wouldn't say she IS awful, because she's been dead awhile now. I would say there was a time when she WAS awful, when she said gay people were demonic and that the people who loved her books would burn in hell. She eventually got over that. She did write a LOT of pedophilia storylines, though, and not in a negative light.
Also, have you read her books now that you are older? I don't think they age well.

I like Diane Duane's books!