I was hoping to get some good hypothetical insults from the realm of film and literature. It’s the mod’s discretion, of course.
Have a look at this:
http://www.ultimateflame.com/
I kind of liked a recent one by Paula Abdul, putting down Simon Cowell on ‘American Idol’: ‘That’s what happens if you get breast fed by your father’.
My favorite insult came from a friend, Mike. We were at the American College Theatre Festival in Hayward, and one of the shows being presented was a deplorable piece of tripe called Butoh Macbeth. It was supposed to combine a Japanese performance art with… well… the story of Macbeth. What resulted was a bunch of actors in actresses, wearing little more than loincloths and bras (for the ladies), covered in white powder, writhing around in agony, occasionally shouting out “Pain!” or “Torment!”
Afterwards, as we were leaving, Mike exclaimed in the most pained voice I’ve ever heard: “That hurt… Theatre’s not supposed to hurt!”
Man, that show sucked ass.
The version I’ve always used is, “I’ve been called worse by people who knew what they were talking about.”
Maybe not high-culture but… Monty Python and the Holy Grail:
“You empty-headed animal food-trough wiper!”
“Your mother was a hampster and your father smelt of elderberries!”
“Now go away or I shall taunt you a second time.”
"The biggest difference between you and I is that people will cry at my funeral!
No one has mentioned “no-talent ass clown” yet?! Seriously, I’m surprised. Office Space is the best. 
Tanaqui
From my junior high school days:
If brains were gasoline, you wouldn’t have enough to power an ant’s mini-bike 'round the inside of a Cheerio.
Q: “What’s the difference between a lawyer and a sperm?”
A: “A sperm has a one in a hundred million chance of becoming human.”
“She gave you a dirty look!”
“Who?”
“Mother Nature!”
“Now I know why some animals eat their young.” - Tommy Smothers
One of the first web pages I ever saw, way back when we were using wood-burning web servers, was a collection of classic insults and put-downs that the page’s creator had been collecting on Usenet for years. Unfortunately, the page has gone to that great 404 in the sky, a sad loss to the world. And I say this not just because one of my gems was listed, oh no. The page was a thing of beauty, a work of art, and a fine way to waste the better part of one lunch hour.
(“If you were the last man on earth, I’d build myself a vibrator.” If anyone’s curious.)
We were once talking about a coworker who, while being a nice guy, is a terrible liar, to the point where we wonder whether or not he actually believes some of the things he says.
One coworker said about him: That guy needs some serious help.
My reply: I don’t think there anything that can cure what’s wrong with him except a bullet.
And while talking about people that really piss us off my mother said: Well they say it takes all kinds to make the world go round.
My reply: Well it may take all kinds to make the world go round, but I wouldn’t mind the world going a little slower if we could get rid of a few of them.
And yes, the Office Space no talent assclown comment is classic.
How could anyone forget:
“If ignorance were cornflakes, you’d be General Mills.”
This column incorporates one of my personal favourites, though I’ve never actually used it in anger:
“I wouldn’t piss on him if his piles were on fire.”
The problem being, of course, that “piles” in this context has a meaning most North Americans don’t get, or at least I didn’t at first.
My own creation follows (warning - profanity):If you weren’t such a cunt I’d feel sorry for you.I’m very proud of it.
In the context of bad theater, one I’ve heard is “The comedy is that they charged for it. The tragedy is that we paid for it.”.
I’d previously heard Eve’s contribution as a comeback, where it works much better. “You know, without your glasses on, you don’t look half bad.” “You know, without my glasses on, neither do you.”
From Good Morining Vietnam, “You’re in more dire need of a blow job than any man in human history.”
“Here’s a dollar - buy a clue”
“(its name here) - for fifty bucks and a shot at the Festiva - how many brain cells DO you have?”
(Rita to Runt, Animaniacs, copyright warner bros, produced by s. speilberg) (mods happy?)
In one of the Flashman books, when referring to a late and unlamented colleague, an Afghan hillman said :
“Abdul Karim, may wild hogs mate above his grave, …”.
(The name wasn’t Abdul Karim, but I’ve long forgotten it).
Here is a site for all the insults you will ever need.