This YouTube compilation is pretty good: http://www.youtube.com/watch?v=PSEYXWmEse8

My favorite will always be the principal from Billy Madison: "Mr. Madison, what you've just said is one of the most insanely idiotic things I have ever heard. At no point in your rambling, incoherent response were you even close to anything that could be considered a rational thought. Everyone in this room is now dumber for having listened to it. I award you no points, and may God have mercy on your soul."

You can see that on YouTube here: http://www.youtube.com/watch?v=5hfYJsQAhl0

May I also suggest character introductions as a theme? It's an important and difficult part of every script and I'd love to see some good ones.

End all, be all favorite...

Clark Griswold needs some Tylenol.

http://www.youtube.com/watch?v=WEE8mNHxWFU

So many classics from Tombstone, but this is one of my favorites:

Doc Holliday: "You know, Ed, if I thought you weren't my friend...I just don't think I could bear it."

http://www.youtube.com/watch?v=ly-9MDqMAd0

Speaking of insults, here is some interesting info about the Sergeant Hartman rant from "Full Metal Jacket" - directly from Stanley Kubrick himself.

"I'd say fifty percent of Lee's dialogue, specifically the insult stuff, came from Lee. You see, in the course of hiring the marine recruits, we interviewed hundreds of guys. We lined them all up and did an improvisation of the first meeting with the drill instructor. They didn't know what he was going to say, and we could see how they reacted. Lee came up with, I don't know, 150 pages of insults. Off the wall stuff: "I don't like the name Lawrence. Lawrence is for faggots and sailors."

This comes from a Kubrick interview from a 8/27/1987 issue of Rolling Stones magazine.

The full interview can be found here:
http://www.rollingstone.com/culture/news/the-rolling-stone-interview-stanley-kubrick-in-1987-20110307?page=1

http://www.youtube.com/watch?v=KuStsFW4EmQ

Monty Python and the Holy Grail never gets old for me...

French Soldier: I don't want to talk to you no more, you empty headed animal food trough wiper. I fart in your general direction. Your mother was a hamster and your father smelt of elderberries.

http://www.youtube.com/watch?v=A8yjNbcKkNY

Hudson; Hey Vasquez, have you ever been mistaken for a man?
Vasquez: No. have you?

Classic put-down. :)

Bah! I've got an excellent quote, but can't find the video for it.

The film is Get Shorty, and the quote is:

Bo Catlett: You think we watch any of your movies, Harry? I've seen better film on teeth.

The Princess Bride - "To the Pain"

This scene contains several memorable barbs, my personal favorite being, "I'll explain and I'll use small words so that you'll be sure to understand, you warthog faced buffoon."

http://bit.ly/eAg4kD

Well, I think you could basically post most of Melvin's (Jack Nicholson) dialogue fromAs Good As It Gets. But, this one hits me where it hurts:

http://www.youtube.com/watch?v=6Y9BukEBI9c

How do you write women so well?

I think of a man and I take away reason and accountability.

IMDB says "Udall's response to the question about how he writes women is an actual response given by author John Updike when asked the same question. "
trivia link: http://www.imdb.com/title/tt0119822/trivia?tr=tr0779286

John Updike?!?

more trivia:
"At the 1998 Oscar ceremony, when his name was announced as the winner of the Best Actor award, Jack Nicholson ascended to the podium by carefully stepping over the cracks between the tiles on the stage, in imitation of his character, Melvin Udall."
http://www.imdb.com/title/tt0119822/trivia?tr=tr0779274

My actual favorite insult in this movie, is the verbal thrashing that Frank Sachs (Cuba Gooding Jr) delivers to Melvin, "My grandma's got more attitude than you," but I couldn't find video for it.

Because A Fish Called Wanda should always get a shout out re insults, here is Otto being frank with Archie:
Otto: You pompous, stuck-up, snot-nosed, English, giant, twerp, scumbag, fuck-face, dickhead, asshole.
http://www.youtube.com/watch?v=5YKbYLb5GVc
(3:46 on clip)