I agree with Johan from Monster and of course Sephiroth. Both excellent villains in my book.

And why hasn't Ganondorf been mentioned? He's a sinister guy too.

Sephiroth, villain who manage to at least kill a single main character, in an RPG game this is a fantastic achievement, not to mention he is someone you can relate to easily, since he and cloud is easily two side of a coin.

Sauron, from the Lord of The Rings trilogy. Even without his one ring he still kicks some ass and actually almost impossible to kill aside from destroying the ring.

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Need a loving and lovely tsundere wife, anyone interested? >.<

On second thought yandere would also be fine.... O.O

Take the Magic: The Gathering 'What Color Are You?' Quiz.

Movie: Darth Sidious, guy manipulated and killed virtually all the jedi and ruled with a iron fist for 20 something years and he comes back and turns luke to the dark side D=
Novel: Lord Voldemort (im a harry potter nut XD)
Graphic Novel: The Joker
T.V. Sylar from Heroes
Manga/Anime: hmm im probably gonna pick Light

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"when i'm sad, i stop being sad and be awesome instead."
- Barney Stinson

TV Show: Dexter (no, not Dexter Lab, lol). I guess he's a hero in the aspect that he kills criminals; however, realistically looking at it, he's killing to soothe his urges. He's complex and interesting, making him one of my picks.

Movies: Like may others, I vote The Joker. I am doing so not because Heath Ledger did a stellar job as him. It's just the principle of the character. A criminal who desires nothing but to cause chaos.

Anime/Manga:
Griffith (Berserk)... God I hate this guy. betraying his comrades, raped his former best friend's lover, and causing so much chaos... despite all of that, everyone think of him as a hero.

Aji Tae (Shin Angyo Onshi) for pretty much the same reasons as Griffith. When you use your former friend like a puppet, take his lover(s), and still act as the good guy... that's just a whole new level of douchebaggery.

Joker from Batman & Sephiroth from FF

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Films
Montag the Magnificent in The Wizard of Gore
and of course The Joker
and The Blob
Mangas
Fuuma in X
Light in Death Note
Uruhi Shalma from Red river
Books
Dracula from Bram Stoker