American Psycho/Funny

The business card comparison scene (especially in the movie) is pretty hilarious:

[Looking at Paul Allen's business card]
Look at that subtle off-white coloring. The tasteful thickness of it. Oh my God, it even has a watermark!

- From the same scene: "...we also go to the same barber, although I have a slightly better haircut."
Bateman's red snapper pizza rant in the book is pretty funny too, especially since it comes totally out of left field:

"No one wants the fucking red snapper pizza! A pizza should be yeasty and slightly bready and have a cheesy crust! The crusts here are too fucking thin because the shithead chef who cooks here overbakes everything! The pizza is dried out and brittle!"

- And then later, when Bateman is forced to eat his words because Donald Trump says he likes the pizza in question.
"Could you keep it down? I'm trying to do drugs!"
The scene in the film in which Bateman reels off all manner of memorized observations regarding the musical stylings of Huey Lewis and The News - all the while preparing to murder Paul Allen.
- Try not to laugh when Bateman moon-walks behind his drugged victim. Also: "Hey, Paul!"
The Chainsaw Scene, especially the part where he drops the chainsaw down a flight of stairs onto her.
- For those who haven't seen the movie, he actually tries tracking his target from above by holding the chainsaw out with one arm and circling it around and around.
The cash machine that says "Feed me a stray cat".
A deleted scene features Patrick painfully trying to be hip and familiar with a Rastafarian, calling him "Marley" and saying "...yeah, we be jammin'..." to himself after being brushed off.
"We're trying to get high off of it, not sprinkle it on our fucking cornflakes!"
"Their lyrics are as positive and affirmative as anything I've heard in rock. Christy, get down on your knees so Sabrina can see your asshole... Phil Collins' solo career seems to be more commercial and therefore more satisfying, in a narrower way. Especially songs like In the Air Tonight and Against All Odds - Sabrina, don't just stare at it, eat it."
The chapter in the book where Bateman and his associates are trying to organize a dinner in a restaurant using conference call and mess everything up.
Bateman's surprised look when he shoots at a police car and it explodes.
'I quickly stand, brushing him off, and and when I think his outburst has subsided and he's able to walk away, Luis grabs my right ankle and tries to hold on as I'm leaving Barney's and I end up dragging him along for six feet before I have to kick him in the face, while smiling helplessly at a couple who are browsing near the sock department.'
There's a scene in the movie where Bateman is making ridiculous faces as he flexes his muscles at himself in the mirror, while he's having sex with a prostitute, as Phil Collins's "Sussudio" plays. This troper still can't listen to that song now without bursting into laughter.
In the first chapter of the book, Timothy Price (Patrick's associate) is reading a newspaper and commenting on all the terrible headlines:

"In one issue... in one issue... let's see here... strangled models, babies thrown from tenement rooftops, kids killed in the subway, a Communist rally, Mafia boss wiped out, Nazis, baseball players with AIDS, more Mafia shit, gridlock, the homeless, various maniacs, faggots dropping like flies in the streets, surrogate mothers, the cancellation of a soap opera..."

Again from the book, there's a fake TV talk show (think Maury Povich) that Patrick occasionally watches called The Patty Winters Show. The topics he mentions start out as pretty normal and mundane (Autism), but they quickly descend into nonsense ("Dwarf Tossing" "A boy who fell in love with a box of soap", and "UFOs That Kill"). Later on, when Patrick's lost all touch with reality, the show has Big Foot on as a guest, who Patrick claimed was "surprisingly articulate and charming", and finally, an apparently anthropomorphic Cheerio.
In the movie, when the hooker is trying to escape from Bateman's apartment, she trips and falls, and Bateman catches up with her... only for him to stop, grab her leg, and start biting on it. She stares at him numbly for a moment, then slowly draws her other leg back and kicks him square in the face. In what should be a horrifying scene, this troper laughed his ass off at that part.