I have to go with Ledger's Joker as well - the character was totally amoral, created chaos without rhyme or reason, he was, literally, just a 'force of nature' who left a path of destruction.
There's also the equally dangerous vilains - those who truly believe tey are doing something right. Kathy Bates in Misery, Alex Forrest (sp?) in Fatal Attraction, Rebecca DeMornay in The Hand........
I loved John Travolta's charcater (Gabriel) in Swordfish for his totally twisted moral compass.
For non-human characters, the Alien in Alien.
As for TV? Tough one. Have to go with Angelus and Faith in Buffy (nobody else in the series hit Buffy as close to home as these two - the lover turned bad and the dark reflection of herself); Brother Cavill in Battlestar Galactica; Robert Carlyle in Cracker; and Glenn Close in Damages (for whom everything and everyone is a means to an end).