What villain, in the realm of fiction, is the most villain-like in various categories? What villain is the most outright evil? The most adversarial? Which has the most makes-you-want-to-reach-into-their-world-and-strangle-them-ness?
Well, the first area I thought of when I read this was comics so, I’ll do that first. Thanos, of marvel comics, was the quintessential villain, and yet I like him more than any other. Now, evil… Randall Flagg, Richard Freemantle, RF, in the Stand (Stephen King) is basically Satan’s right-hand man, and is the epitome of evil. Basically, you love to hate him. I’ll be back if more pop into my head
RUNNERS-UP::
Witchiepoo (H.R. Puffnstuff)
Sleestaks (Land of the Lost)
Yosemite Sam (Warner Bros.)
Marvin Martian (Warner Bros.)
Dick Dastardly (Penelpoe Pitstop, Wacky Races)
Bluto (Popeye)
Gargamel (Smurfs)
J.D. “Boss” Hogg (Dukes of Hazzard)
Skeletor (He-Man)
The Imperious Leader (Battlestar Gallactica)
J.R Ewing (Dallas)
Conrad Von Siegfried (of KAOS, Get Smart)
Newman (Seinfeld)
Buddy Hinton (who teased Cindy because she had a lithp, Brady Bunch)
THE WINNER:
COBRA COMMANDER (G.I. Joe)
Honorable Mention:
Baron Otto Matic (Tom Slick)
Boris (Rocky & Bullwinkle)
The Professor (Felix the Cat)
What about Sauron? No redeeming qualities whatsoever, except perhaps his insatiable appetite for fine Corinthian leather.
I thought immediately of Kevin Spacey in Seven.
He was one evil fucker!
Emperor Palpatine. Damn, but he had some good lines in ROTJ. (“Oh, I’m afraid the deflector shield will be quite operational when your friends arrive.” I still get shivers when I hear him say that.)
KAAAAAAAAAAAHHHHHHHHHN!..
KAAAAAAAAAAAHHHHHHHHHN!
Nurse Ratchet; One Flew Over the Cuckoo’s Nest .
Sephiroth from Final Fantasy VII. Casually murders anyone who gets in his way, manipulates and mind controls at a whim, intends to massacre everyone on the planet to obtain godhood for himself, and too mean to stay dead.
The fat rich kid from Pee Wee’s Big Adventure.
Well, him or Satan in the Exorcist. You tell me which one is more evil. 
One of my favorite villains was from a book which I can’t remember the name of right now, it was one of the Warhammer Fantasy series books. The defining moment for this guy was when he killed someone and giggled about it. Not laughed in triumph, not gloated over it or did it casually, but offed a man and giggled like a little schoolgirl.
Are you kidding? Sephiroth was a friggin’ Boy Scout compared to Kefka.
The quintessential villain: Insane, but understandably so. He had a direction, a method to his madness, and wasn’t just entirely random. A genius, and incredibly powerful and devious. But what’s more… his villainy wasn’t something that he did himself, but was something that was thrust upon him.
And he had a great laugh. Bwahahahahaha!
And his wardrobe kicked butt. And he wasn’t afraid to wear fruity makeup. http://sinclair.weyland-yutani.net/art/ff6kefka.jpg
I think Scorpius from Farscape is incredibly evil, but he needs to be around a while longer before he can really be considered an all-timer. He’s hideous, but highly intelligent and smooth-talking. He enjoys torturing others. He kills almost absent-mindedly. And the kicker: he ate a piece of our hero’s brain. What more do you need?
SPOOFE
Aaaaahhhh! Evil Clown!!!
The first one to come to mind that met all of these criteria is Archibald Cunningham.
who?
Archibald Cunningham, perfectly played by Tim Roth in the movie Rob Roy, starring Liam Neeson in the title role.
Never before have I so badly desired to kill a fictional character with my bare hands.
Damn. 15 responses in 12 hours. Thanks, people.
Iago from Othello: Knows that bringing down Othello will destroy him as well but just doesn’t care.
Judge Holden from Cormac McCarthy’s Blood Meridian: giant, hairless, omniscient, polyglot, child-abusing ( and murdering) albino. Seems to regard the rest of the human race as playthings for his own amusement.
On a lighter note.
Count Olaf: arch-enemy of the Baudelaire orphans in Lemony Snicket’s Series of Unfortunate Events. Perpetrator of the most ingenious plans ( and wearer of the worst disguises) in children’s fiction.
Iago was the first to come to mind, but I sort of get the impression that he’s more a parody of a villain, than a villain himself. That scene where he’s telling his motives? It comes across, to me, as sort of “Well, I’m the villain, so obviously I need some reason to hate Othello. Hmm… Let’s see… How about, I think he’s having an affair with my wife? Yeah, that’ll work.”. For a Shakespearean villain, I’m going to have to go with either Shylock or Macbeth.
And Sauron, bad as he is, still doesn’t compare to his mentor and former master, Melkor who is called Morgoth. Even he’s not my pick for a Tolkien villain, though: That would have to be Ungoliant, the primordial Spider of Darkness, who craves light and yet hates it. The more beauty she devours and excretes in her foul webs, the hungrier she becomes, until, legend has it, she ate herself.
If you’re going with Shakespeare villains, I’d have to pick Lady Macbeth.