The Vibrating Luggage conversation. The narrator's insistence that he doesn't have a dildo in his bag is funny enough, but the guy behind the desk's response - a look that screams "whatever you say, pal" - tips it into comedy gold.
When the snooty cat and the courageous dog with the celebrity voices meet for the first time in reel 3. Not to mention Tyler's Aside Glance into the camera with a genuine smile.
Tyler: (sweetly) A nice. Big. Cock.
Marla's quote after the first time she has sex with Tyler. Sure it Crosses the Line Twice, but it's funny as hell. What makes it even funnier is that Helena Bonham-Carter actually had no idea how shocking it was - since they don't use the term grade school in England she thought the line was fairly innocuous, and was horrified when someone explained it to her later.
The deleted take's line is both horrifying yet hilarious at the same time: "I wanna have your abortion".
"I am Jack's smirking revenge."
The scene near the end with Tyler, the narrator and the van full of nitroglycerin definitely qualifies, even if it's also one hell of a skin-curler.
Tyler: OKAY! You are now firing a gun at your imaginary friend NEAR 400 GALLONS OF NITROGLYCERIN!
You've gotta be pretty balls-to-the-wall crazy to have your hallucination call you nuts. It's kinda cool since it shows that the symptoms "Jack's" schizophrenia isn't just the existence of Tyler, it's more realistic than many Hollywood depictions.
A subtler one: when the Narrator shows up at the building near the end, and Tyler's there and tells him that he looks "like a crazy person".
The "nice big cock" reappearing just before the end credits.
Tyler wearing a rubber glove after having sex with Marla.
Even funnier is that David Fincher said he was dying of embarrassment when that scene was shown at the premiere, knowing that Helena Bonham Carter's mother was in the audience. Fortunately, she apparently thought the scene washilarious.
The guy trying to turn away one of the applicants for Project Mayhem can't come up with anything to shout at him, except "And you are too fucking... (beat)... BLONDE!" It's probably the Large Ham delivery.
When Tyler gives out his first "homework assignment" to fight a total stranger, and the members of Fight Club start chasing down random people on the street, with many of them running away.
Especially when one of them tries to goad a priest into fighting him by spraying him and the bible he's carrying while it lies on the ground with a hose, and it actually works. If you notice, the camera shakes a bit during the priest scene. The camera man was laughing too hard to hold it steady. And if you look carefully later in the movie, you'll see the priest as a member in the fight club.
Not to mention the car salesmen. When he's pushed to the ground, his shoe goes flying up in the air. That, and his high pitched "Son of a bitch!" sell the scene.
The Narrator's Breaking Speech to his boss, and later on when he beats himself up in his boss' office.
The scene in question had the potential to be extremely unsettling, but having the narrator whistling cheerfully while the security guards escort him out of the building rather undercut that.
"Self improvement is masturbation." Hot damn, Tyler, that is both horribly funny and insightful.