Teddy: CHAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAARGE! CHARGE THE BLOCKHOUSE!!!!
Teddy's encounter with Jonathan:
Jonathan: "Go to bed, Teddy."
Teddy: "Who are you?"
Jonathan: "I'm Woodrow Wilson. Go to bed."
Jonathan slowly coming to the realization that he and his kindly old aunts have murdered the same number of people.
Especially the ensuing "score tallying" scene:
Dr. Einstein: Johnny, don't brag! You got twelve, they got twelve; the old ladies is just as good as you!
Meanwhile the aunts are looking quite flattered.
And of course, his frustration that while he's been hunted like a dog for years due to his murders, his aunts have never been caught and are still living their happy lives.
The aunts' latest "charity case" is handed a glass of the elderberry wine while Mortimer tries to reach the sanitarium on the phone. After hanging up, he distractedly grabs a glass of his own. The aunts warn him off, he realizes what he's doing and puts the glass down...and seeing the visitor about to drink, lets out a piercing shriek of horror.
"Insanity runs in my family. (beat) It practically gallops!"
Mortimer: (singing) "There is a Happy Dale far, far away!"
Dr. Witherspoon's reaction to it is just as funny:
Lt. Rooney: Look here, doc, couldn't you also make room for the ladies (at Happy Dale with Teddy)?
Dr. Witherspoon:Just the ladies?
At one point during Mortimer's rant at the top of the stairs, Teddy comes out of his room, watches him for a while, and then goes back in making a "he's screwy" gesture.
Mortimer's increasingly funny Sanity Slippage. He goes from suave, assured on-top-of-the-world Only Sane Man, then discovers he's the Only Sane Man (because for that to happen, there's the rather unpleasant discovery that everyone else is insane, including all of his family, and it may be hereditary), and the rest of the movie is basically him not dealing well with that information while managing about three or four plotlines at once. By the end, he's been morphed into a high-strung, Seen It All man who's over the moon to find out he's not in hereditary danger of a mental disease, but may have been driven to one anyway.
Teddy's initial reaction to meeting Dr. Witherspoon:
Teddy: Is he trying to move into the White House before I've moved out?
Mortimer: Who, Mr. President?
Teddy:(points accusingly at Witherspoon) Taft!
Witherspoon:(indignantly sucks in his gut)
After being confronted by her nephew Mortimer about the twelve men she and her sister have murdered, Aunt Abby huffily informs Mortimer that she "would never stoop to telling a fib!"
Dr. Einstein's exit, where he quietly and nervously makes his escape while everyone else seems completely oblivious about what he's doing.
Aunt Martha: Dr. Einstein! Are you leaving us?
Dr. Einstein: Yes, please.
One of the cops even reads of a description of him from his wanted poster while he's standing right there without realizing it.
Culminating in his nervous little laugh as he departs. It really must be seen to be appreciated.
The whole fight between Jonathan and the cops, which Mortimer watches disinterestedly from the safety of the stairs.
Just after Mortimer has successfully committed Teddy and his aunts to the asylum without revealing they have murdered a dozen people, Elaine discovers the bodies and runs up screaming, nearly blowing the whole thing. Mortimer shuts her up kissing her repeatedly and carrying her out of the house.
Lt. Rooney: "What's wrong with her?"
Mortimer: (still muffling her screams with his lips) "It's way past her bedtime!"
The end line, as Mortimer is making out with Elaine:
Cab Driver: But Mr. Brewster...!
Mortimer: Oh, no! I'm not a Brewster! I'm the son of a sea cook! (throws Elaine over his shoulder and runs down the street) CHAAAAAAAAAAAAAAAAAAAARRRGE!
Cab Diver:(calling after him while holding his arms in the 'tea pot' pose) I'M NOT A CAB DRIVER I'M A COFFEE POT!
Jonathan has Mortimer tied up and gagged and is about to torture him to death when Dr. Einstein finds the poisoned wine and suggests a toast. Just as he and Jonathan are about to drink, Teddy blows a loud bugle signal outside, making them both jump and drop their glasses... and Mortimer goes right back to looking terrified.