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