Who is the most frightning villian (any medium)?

I think that the Weeping Angels from "Doctor Who" may take the cake on that. Beings that can only attack you when you aren't looking. When you are they are stone statues. And your only defense is to not blink or they will steal you very time from you. Also "The Siant of Killers" from the "Preacher" comic series gets mad props for being nigh immortal and having six-shooters that can kill the devil and god.  
http://tardis.wikia.com/wiki/Weeping_Angel    

The Gentlemen off Buffy. 
 

Yeah i love the angels as well, but it's kind of silly that they don't just walk over to them, pick them up, and drop them off a cliff. As long as they look at them they can't do a thing!  

do animals count? cause the librarians from metro 2033 are pretty frightening

GLaDOS. Because I love her but she is willing to kill me for science. 

wtf is all this shit? weeping angels? gentlemen? no. wtf. 
 
It's The Thing. Without a shadow of a doubt, the most terrifying villain ever created.

" @The_A_Drain: Wait.. which one? You mean from the horror movie, The Thing, or The Thing from comics? Specify. "

That freaking octopus in my dreams... I really wish he would leave me alone.

"That freaking octopus in my dreams... I really wish he would leave me alone. "

No idea, but Nightmare from Metroid Fusion comes to mind:
 

The Darkness from The Darkness.

" @h0lgr said:

" @The_A_Drain: Wait.. which one? You mean from the horror movie, The Thing, or The Thing from comics? Specify. "

Well, which one do you think is scarier? O_o He might clobber people from time to time but I don't think Marvel Comics Thing is scary :P  Specifically, I mean the alien entity from both the John W Campbell novella "Who Goes There?" and the John Carpenter remake of The Thing (from another world, the Howard Hawkes version is shit tho imo)  I mean, just think about it. That thing could even be you, and you'd never know! Until it's too late. "

The Strangers from Dark City.

" @masterpaperlink: you mean Cthulhu? "

yeah...... more like this...

" @The_A_Drain said:

" @h0lgr said:

" @The_A_Drain: Wait.. which one? You mean from the horror movie, The Thing, or The Thing from comics? Specify. "

Well, which one do you think is scarier? O_o He might clobber people from time to time but I don't think Marvel Comics Thing is scary :P  Specifically, I mean the alien entity from both the John W Campbell novella "Who Goes There?" and the John Carpenter remake of The Thing (from another world, the Howard Hawkes version is shit tho imo)  I mean, just think about it. That thing could even be you, and you'd never know! Until it's too late. "

So you're telling me i should go back and watch it? Cuz i watched for 20 mins and just... stopped. it was boring. They went into a cave and meh..  The only reason i tried to watch it is because of the lice episode from South Park "

Frank Booth/Dennis Hopper
Blue Velvet
If you like strange, disturbing stuff.

Sovereign

I would say anything that can look like a human or inhabit one.

Freddy Krueger 
  

One, two, Freddy is coming for you two.

Three, four, better lock your door.

Five, six, grab your crucifix.

Seven, eight, better stay up late.

Nine, ten, never sleep again.        

David Bowie as the Gobling King in Labyrinth.

Uncle Frank, from Hellraiser.  Unfortunately, the sequels, which Clive Barker didn't create, centered on Pinhead...  a shame... 
 

Freakin' children, THIS GUY is the most frightening villain in movies. Because he's so subtle and calculating. You can never understand him. He can sit next to you on a plane and you would never notice him. He is the true embodiment of evil.

Sorry.
 

I think that the Trinity Killer from Dexter and Jigsaw from the Saw films (the first 3 at least) are really good villians. 
 
Trinity Killer because he is a sick twister m'er f'er hiding in plain sight and Jigsaw because he is a decrepit old dude who, again is a twisted fucker, but is behind the scenes in complete control of ever situations.  
 
Also, I am pretty young, so I'm pretty ignorant of a lot of the best films and TV shows.

Nemesis.
 
No other character in video games has scared the shit out of me as much as this guy.  First time I met him I ran away through a door.  Was sitting there inside the Police Station wondering what he was when he followed me through the door.  Fucking terrifying.
 
 

From Firefly the Reavers were pretty scary, although it's the Glove Men who take the cake. The way they just kill without violence or feeling, and have the backing of a massive Government is scary.

Time, from Reality.

The scariest things are those we see everyday.   After seeing The Birds, every time I see a telephone line with a long line of migrating birds, I get the "willies".

The scariest thing is the one that you know not of,not the one you can see  and predict.