Lo Pan from Big Trouble in Little China. “Now this really pisses me off to no end!”
Mark Rydell Marty Augustine in The Long Goodbye. He’s so psychotic and unpredictable, he’s chilling.
John Glover as Alan Raimy in 51 Pickup. Also very unpredictable.
Rutger Hauer as Roy Battie the replicant in Blade Runner.
[spoiler]He leads a renegade band to slaughter the crew and passengers of an offworld shuttle to reach Earth, a planet forbidden to their kind, then kills several more “innocent” people (guilty only of complicity in making beings like him), all in order to reach an audience with his maker and demand to his face: I want more life.
And when he’s told it’s impossible, he kills his “Father”.
Hunted down like the dangerous psychopath he is, at the end, his dying act is to save his pursuer (who has just killed his replicant girlfriend, causing him to experience grief for the first time) from falling to his death.
Time… to die.
[/spoiler]
Hard to top!
I echo the calls for Alan Rickman in Die Hard and Robin Hood
“and bring a friend!”.
I liked Rutger’s replicant in Blade Runner, but he seems more tragic to me than evil.
“I’ve seen things you people wouldn’t believe. Attack ships on fire off the shoulder of Orion. I watched C-beams glitter in the dark near the Tannhauser gate. All those moments will be lost in time, like tears in rain. Time to die”.
Jack Nicholson, as The Joker.
Lots of good ones mentioned. I’d add the animated Smaug. “I kill where I wish and none dare resist !”
“My armor is like tenfold shields, my teeth are swords, my claws spears, the shock of my tail a thunderbolt, my wings a HURRICANE . . . and my breath, death.”
With plenty of visual aids in the movie.
Hal 9000
Oooh.
Godzilla.
I win.
The head slave raider in Apocalypto.
Crooked DEA agent Norman Stanfield (Gary Oldman) in Leon (a/k/a The Professional) – capable of delivering an erudite lecture on classical music even while he’s coming to kill you.
Sorry, Rickman rocks. (We’ll win on the vote count.)
The Thing, from The Thing. Godzilla gets Thingified.
I win. 
More of an antivillain – victim of circumstances, non-human psychology, etc.
No, no – a kaiju is not a villain, it is a force of nature.
So many villains to consider… and so many kinds of movie villainy!
In the Cartoony Evil bracket, I have to go with Ernst Stavro Blofeld, he of the ever-morphing physiognomy and other quirks (monocle, bejewelled Persian cat, grey Nehru jackets, surgically altered body doubles, Mike Myers knockoffs…).
In the Serious Evil bracket, there’s one of Vincent Price’s many screen villains particularly worth a mention: his Matthew Hopkins in The Witchfinder General [1968], based on the 17th-C. English witch-hunter. Hopkins actually applied the floating/drowning test to detect witches, as lampooned in Monty Python & the Holy Grail, as well as tortuous pricking to find “devil’s marks,” etc.
And in the Hannah Arendt’s Banality of Evil bracket, I really hated the cowardly, balding white dude (Harry Cooper, played by Karl Hardman) in The Night of the Living Dead, who absolutely refuses to do anything useful to fight off the zombies. His loathsome personality alienates everyone in that house, including his wife, who probably had no idea that he was that pathetic and despicable prior to that night. His hogging the relative safety of the basement prefigures a similar move by Paul Reiser’s Burke in Aliens.
I just wanted to add the reavers from serenity/firefly. Not as sophisticated as Alan Rickman’s characters, but they do have a certain barabarian “charm”.
I just saw Blade Runner recently for the first time (after several attempts over the years where I got bored or fell asleep). He is both tragic AND evil, but that is a hell of a speech to go out on.
I’ll also go for:
Alan Rickman as Hans Gruber in Die Hard
Gary Oldman as Stansfield in Leon and Drexl (a terrifying pimp) in True Romance
Anthony Hopkins AND Brian Cox as Hannibal Lecter in their respective movies
Lance Hendricksen and Arnold Vosloo as the villains who were hunting homeless Vietnam vets through New Orleans in Hard Target
Does TV count? I’d really like to mention Powers Boothe and Gerald McRaney as Cy Tolliver and George Hearst, in Deadwood. Both villains you love to hate.
As a villain? In 90% of those movies, he’s the hero.
Destructiveness: Gozer the Gozerian of Ghostbusters.
Heartlessness: The T-1000 of Terminator 2.
Flamboyant Evil: Jafar of Aladdin.
Alan Rickman? Hah! Gary Oldman’s villains beat his bad guys hands down.
Hans Gruber vs. Drexl Spivey from True Romance? Drexl would own Gruber’s teutonic buttocks in one round (probably literally). Clarence Worley got lucky, that’s all.
Stansfield from Leon rocked too. I actually liked him better than Leon, because Stansfield might have been a corrupt cop, a druggie, and a murderer, but at least he wasn’t a pedophile. That counts in my book, ya’ know?