This one cracks me no matter how many times I read it.
A furious light sabre duel is under way. DARTH VADER is backing LUKE SKYWALKER toward the end of the gantry. A quick move by Vader chops off Luke's hand! It goes spinning off into the ventilation shaft. Luke backs away. He looks around, but realizes there's nowhere to go but straight down.
Darth Vader: "Obi Wan never told you what happened to your father."
Luke: "He told me enough! He told me you killed him!"
Darth Vader: "No! I am your father!"
Luke: "No, it's not true! It's impossible."
Darth Vader: "Search your feelings; you know it to be true."
Darth Vader: "Yes, it is true and you know what else? You know that brass droid of yours?"
Darth Vader: "Yes, Threepio, I built him when I was seven years old."
Darth Vader: "Seven years old? And what have you done? Look at yourself, no hand, no job, and couldn't even levitate your own ship out of the swamp."
Luke: "I destroyed your precious Death Star!"
Darth Vader: "When you were 20! When I was 10, I single-handedly destroyed a Trade Federation Droid Control ship!"
Luke: "Well, it's not my fault."
Darth Vader: "Oh, here we go. 'Poor me, my father never gave me what I wanted for my birthday, boo hoo, my daddy's the Dark Lord of the Sith .. waahhh wahhh!'"
Luke: "Shut up."
Darth Vader: "You're a slacker! By the time I was your age, I had exterminated the Jedi knights!"
Luke: "I used to race my T-16 through Beggar's Canyon!"
Darth Vader: "Oh, for the love of the Emperor, 10 years old, winner of the Boonta Eve Open. Only human to ever fly a Pod Racer, right here baby!
Luke looks down the shaft. Takes a step toward it.
Darth Vader: "I was wrong. You're not my kid. I don't know whose you are, but you sure ain't mine."
Luke takes a step off the platform, hesitates, then plunges down the shaft.
Darth Vader looks after him.
Darth Vader: "And get a haircut!"