How the heck did we get this far into the thread without mentioning Dolores Umbridge?
Bette Davis as Madame Sin.
Eve, from the Parasite Eve game.
…and she files her fingernails while wearing gloves.
Veda Pierce in the Mildred Pierce book and film(s). Veda is like a snake - “pretty to look at, but not to take home.”
Amanda Donohoe as Lady Sylvia Marsh in Lair of the White Worm.
Not much of a movie, to be honest. But watching that as a 14-year-old was one of the formative experiences of my life.
Ooh, good one.
I was about that age, too, and it made quite an impact.
Servalan from Blake’s 7.
Princess Ardala in Buck Rogers. (SFW if your employers don’t frown on jewelled space bikinis.)
I came in to post Servalan! Devious, ruthless, and totally stylish, IIRC the character was originally intended to dress in army-type fatigues etc, but Jacqueline Pearce successfully lobbied to have her always in full evening dress or similar. And finally of course, at the end of the final episode (enormous spoiler ahead)
She wins. Blake and all the other rebels all die, after walking into a trap she set. They really do.
Mom, from Futurama.
Poison ivy.
I’m adding these to my list and know there’s at least one other I’m having trouble recalling.
Since I was a wee tyke when I first saw her, and since her beauty was her most dominant characteristic, I probably never really thought about Delilah as played by Hedy Lamarr in Samson and Delilah (1949) as being a villain. With hindsight, though, I think she qualifies for consideration.
Some of the roles mentioned so far are not so much “villains” as well-drawn impressively negative characters. The Matty character in Body Heat (1981) and Livia in The Sopranos (TV Series 1999–2007) are two prime examples. And perhaps for more personal reasons, I can’t elevate the ones in animated productions to the same level as Agent Stahl. They’re good and scary and malevolent, but they’re too gimmicky to impress me with their innate evil.
My main memory barrier to the real contenders for Ultimate Female Villain status is that the TV shows I watch regularly don’t have “villains” as much as strong female characters, and there’s a big difference for me in those types. And perhaps the greatest measure of how villainous a character can be is the amount of pure joy that is brought about by that villainess’s demise such as the way Agent Stahl’s brain splattered on that windshield made me feel. More often than not, the bad gal just goes away or dies more or less peacefully and the catharsis is lacking. It’s rare to see a female’s final moments generate as much reward as Gene Simmons’s did in Wanted Dead Or Alive - head explosion.
Diana Riggs played Mrs. Danvers the house keeper at Manderley House in Rebecca
Alex Forrest (Glenn Close) in “Fatal Attraction.”
Thetis (Maggie Smith) in “Clash of the Titans.”
Catwoman (Julie Newmar or Eartha Kitt or Michelle Pfeffer) from various “Batman” media.
Lucy Van Pelt from “Peanuts.”
Veruka Salt from “Willie Wonka.”
Of all the excellent choices in this thread, she is the one I most want to strangle. She’s evil in so many ways. Of course, I’m sure your referring Angela Lansbury in the original film, not the tepid remake of a few years back.
Hexadecimal from REBOOT
Knowing her, she might just be sharpening her gauntlets. 
Anna May Wong in “Thief of Baghdad”
Jeanine in “This is Spinal Tap”
The best villains are the ones who delude themselves into thinking they’re the good guys. For this reason I nominate Deep Space Nine’s Kai Winn.
Commander Cavillo from The Vor Game.
Lila from season 2 of Dexter. I can’t see Jaime Murray in anything else without supposing that she’s evil to the core.
Gaia on Rome. After she killed Pullo’s wife and unborn child, I was ferociously happy to see him strangle her to death, even though she was already dying.