Dialogue is a weapon. And depending on what you're writing (a novel, a short story, a screenplay, or a play), a great line of dialogue can invade your audience's collective mind and stay there forever.
It can be funny or tragic, illuminate a character or nail a universal human emotion. It can be short or long. There are no rules; it just has to be perfect.
Here are some beauties:
"A boy's best friend is his mother."
Norman Bates/Anthony Perkins in PSYCHO (1960) by Joseph Stefano
"I am big. It's the pictures that got small."
Norma Desmond/Gloria Swanson in SUNSET BOULEVARD (1950) by Billy Wilder and Charles Brackett
"Gentlemen, you can't fight in here! This is the War Room!"
President Merkin Muffley/Peter Sellers in DR STRANGELOVE (1964) by Stanley Kubrick, Peter George, and Terry Southern
"One's too many and a hundred's not enough." — Nat the Bartender/Howard Da Silva in THE LOST WEEKEND (1945) by Billy Wilder and Charles Brackett
"And don't forget: I'm also just a girl, standing in front of a boy, asking him to love her." -- Anna Scott/Julia Roberts in NOTTING HILL (1999) by Richard Curtis
"Oh no, it wasn't the airplanes. It was Beauty killed the Beast." -- Carl Denham/Robert Armstrong in KING KONG (1933) by James Creelman and Ruth Rose (or any of the five other contributors to the screenplay)
"You don't understand! I coulda had class. I coulda been a contender. I could've been somebody, instead of a bum, which is what I am." – Terry Malloy/Marlon Brando in ON THE WATERFRONT (1954) by Budd Schulberg
"I picked you for the job, not because I think you're so darn smart, but because I thought you were a shade less dumb than the rest of the outfit. Guess I was wrong. You're not smarter, Walter...you're just taller." — Barton Keyes/Edward G. Robinson in DOUBLE INDEMNITY (1944) by Billy Wilder and Raymond Chandler
"You can't handle the truth!" – Col. Jessup/Jack Nicolson in A FEW GOOD MEN (1992) by Aaron Sorkin
and one more dramatic ice-pick from a play
"I've had hell inside of me, and I can spot it in others." -- Hickey in THE ICEMAN COMETH (1946) by Eugene O'Neill
Here are some YouTube links:
A FEW GOOD MEN – Jack at his best -- and
ON THE WATERFRONT – classic Marlon
SUNSET BOULEVARD – Gloria Swanson and Billy Wilder – a real peak
I'm working on some of these myself.