I can barely remember anything of “The Return of the Native,” but I did dearly love Eustacia (I hope I got her name right). Never thought she was that bad, just misunderstood 
Salma Hayek, as a vampire in ‘From Dusk till Dawn’. 
As above, a close runner-up is Linda Fiorentino in ‘The Last Seduction’, because she had beauty and brains. 
Nurse Ratched/Baroness Bomburst** (tie)
** Chitty Chitty Bang Bang
Well, Lorelei Lee (Anita Loos’ “Gentlemen Prefer Blondes”) and Belle Poitrine (Patrick Dennis’ “LIttle Me”) could technically be considered either villainesses or heroines . . .
I do love Phyllis Dietrichson of James M. Cain’s “Double Indemnity”—believe it or not, she’s even creepier in the book than in the movie!
Madame Beck from Villette, another chilly character Christabel from Coleridge’s poem. Vivien from Merlin & Vivien.
How can people vote for Mrs. Danvers and not mention Rebecca herself?
But really, the worst has to be the Marquise de Merteuil in Dangerous Liaisons as played by Glenn Close.
[sub]Hmm, Fatal Attraction, Cruella Deville, The Marquise…You know, Glenn may be the winner for playing the most, as well as the most evil villianesses.[/sub]
I may get slapped for this, but one I loved when the goofy Warriors of Virtue came out was the pretty and virginal princess from same. She turns around and hands over her friends and heroes of the world to their worst enemy for youth and power.
Halle Berry’s character from Swordfish. Talk about manipulative!
And yeah, Catwoman.
No offense taken. Sometimes it’s fun to be the baddy.
I’ll throw in Scarlett O’Hara. Very few redeeming qualities as far as I can tell.
Oh, and Vivi from Little Altars Everywhere. She was great in Secrets of the Ya Ya Sisterhood, but I absolutely hated her in Little Altars. If you haven’t read it, don’t. It just wrecks the sequel.
Servalan. Hands down. Especially for her ability to outsmart the protagonists into doing her work for her.
I can’t believe no one’s mentioned Drusilla yet from BTVS. I know I can’t be the only geek around here who thinks she’s the cat’s pajamas.
Also the witch from Hansel and Gretel always seemed pretty freakin’ psycho, what with all that cooking and eating of small children.
bella
OK, I’ll say it if no one will: Callisto from Xena: Warrior Princess.
Hmm. I would have to say the most evil villaness is Martha Stewart …oh wait. She’s not fictional? Darn. 
The Emerald Empress from the Legion of Super-Heroes. She was nasty as a member of the Fatal Five, and even better on her own (this, of course, is prior to the Legion re-boot).
Runner-up: Snow White’s evil stepmother.
Cersei Lannister from A Game of Thrones and Beatrice Lacey seem like birds of a feather. How about Jean Grey when she’s possessed by the Phoenix entity? Shota from Wizard’s First Rule and sequels is sometimes friend, sometimes foe, always bitchy. For Lady MacBeth, I prefer the “Throne of Blood” version by Akira Kurosawa. She’s extra vicious in that movie.
Thought of a couple more ladies to add to my list:
The Borg Queen from “Star Trek: First Contact”. Played by Alice Krige, who does slinky, utterly amoral villainesses well (I must also give Ms. Krige props for her role as Placidia in the “Attila” miniseries) one is reminded of Ebert’s comments about her in his review of “ST: FC”: “The Borg Queen, who looks like no notion of sexy I have ever heard of, but inspires me to keep an open mind.”
Demona from “Gargoyles”. Angry, vengeful, self-righteous, manipulative, she’s lived a hundred life-times without letting go of her hatred. That’s a record, isn’t it?
Dark Phoenix from Marvel’s “X-Men”. She has the power of a god, but the heart of a human. After being mind-controlled, going insane, whipping her teammates, going on a galactic crime spree, and eating a star, she goes out with a bang, so to speak.
.:Nichol:.
Yay! Go Dru go! No one does the crazy evil better than her. I’m relatively new to BtVS, so I’ve only seen a few of her episodes, but she still rocks my world.
Any version of EvilWillow gets a big thumbs up from me, as well.
Rebecca de Mornay in “the hand…” is pretty creepy…