Just about everything Cary Grant/Roger Thornhill says.
"Now you listen to me, I'm an advertising man, not a red herring. I've got a job, a secretary, a mother, two ex-wives and several bartenders that depend upon me, and I don't intend to disappoint them all by getting myself slightly killed. "
Eve Kendall: What happened with your first two marriages? Roger Thornhill: My wives divorced me. Eve Kendall: Why? Roger Thornhill: I think they said I led too dull a life. (Note: this conversation took place while they were hanging for dear life on Mount Rushmore)
Hitchcock having a bus door slammed in his face, literally just after his credit appears on the screen.
The scene in the crowded elevator where Roger's mother asks Vandamm's two henchmen, "You gentlemen aren't really trying to kill my son, are you?" causing an awkward pause. Then the henchmen start fake-laughing, causing everyone in the elevator but Roger to laugh. The topper is when Roger's mother starts laughing, too.
The Newhart Phonecall between drunken Roger, who's been detained at the Glen Cove police station, and his mother.
"No, Mother, I have not been drinking. No. No, these two men, they poured a whole bottle of bourbon into me. No, they didn't give me a chaser."
Then there's his surprised reaction to the police officer's name: Sgt. Emil Klinger.
(After giving the name to his mother) "No, I didn't believe it, either."
While escaping, Roger enters a woman's hospital room through the window. Startled, she exclaims, "Stop!" Then she puts on her glasses, and gets a better look at Roger as he hurries through the room. Entranced, she asks him once again to "stop," this time in a more suggestive tone.
His finger-wagging, "don't start with me!" reaction really sells it.
Vandamm's reaction to Leonard's death is priceless:
Hitchcock himself has a few in the film's trailer:
"Have you planned your vacation yet? You've a choice between sand and sunburn, or mountain climbing and the charley horse. I find it all very enervating, but we should all have some kind of holiday. So, my suggestion is a quiet little tour, say about... two thousand miles?"