H.G. Wells in Warehouse 13.
She gets my vote. Her character was 85% completely evil 
and yet her personality was extremely attractive. And man was she hot. She was one of the few villains I sort of rooted for and wanted to see more of.
Oops, I just realized I made a mistake–Snape doesn’t fit the criteria. Neither does Magneto. But at least I got the Gruber brothers right. 
This is exactly what I was going to say. My all time fav villian is Hans Gruber.
Catherine Zeta-Jones as Sala in The Phantom.
Tia Carrere as Juno Skinner in True Lies.
Rebecca De Mornay as Peyton Flanders in The Hand That Rocks the Cradle
Daryl Hannah as Pris in Blade Runner
I’m a cartoon geek, so:
Dr. Mrs. the Monarch, aka Dr. Girlfriend, Queen Etherea, Lady Au Pair, Sheila
or
Molotov Cocktease, even if she does dress sort of like Pink on her days off.
Both from Venture Brothers, the best show on tv today.
Maggie Q as Mai Linh in Live Free or Die Hard.
Barbara Carrera as Fatima Blush in Never Say Never Again.
Sorry for multiple posts. Names just keep popping into my head. I think it would be easier to start with a list of the hottest actresses and check IMDB for bad girl roles.
Natasha Henstridge as Sil, in Species. Sort of an obvious example, though I’d note that the character is at least as much a victim of villainy as a villain herself. She’s grown as a science experiment, barely escapes execution as a child, and her “transformation” into an adult on the train is absolutely terrifying - you can’t help but feel for that kid.
Have I mentioned lately what excellent taste you have?
From Xena;
Hudson Leick as Callisto.
I’m going even further on the geek scale, and referencing an anime:
Johan, from Monster. This picture’s a bit freaky, but then, he’s a bit freaky. More than a bit freaky.
Azula from Avatar the Last Airbender.
My vote has always been for Michael York as Tybalt. Damn tasty.
Of course, the two leads in that film are two of the most exquistely beautiful humans to ever draw breath. Zeffirelli had an eye for beauty, that’s for damn sure. Hussey is the only actress I can think of that really rises to Shakespeare’s discriptions of Juliet.
Young Kathleen Turner in “Body Heat.” So young, so bad, so what?
From the world of cartoons:
Semi-NSFW – cartoon cleavage, lots of it:
Bomb Queen
Going back a few years: Rene Belloq, from Raiders of the Lost Ark.
Any played by Alan Rickman. I used to have a huge crush on Lee Van Cleef, too.
How is Snape wrong? He’s always struck me as unambigiously bad, and Dumbledore never should have trusted him.
No problem, I forgot one myself: Darken Rahl from Legend of the Seeker is way hotter than the hero.
Dopey hairdo notwithstanding, Javier Bardem as Anton Sigur gives me the shivers in a good way.
