Draco Malfoy, when he's Transfigured into the Amazing Bouncing Ferret.
Harry Potter's secret heartache? What's ailing you now?
Re: Arthur Weasley's eccentricities:
Molly Weasley: Your father thinks very highly of Mad-Eye Moody.
Fred Weasley: Yeah, well, Dad collects plugs, doesn't he?
Divination homework, doing a month's worth of self-predictions filled with every human misery possible. "You seem to be drowning twice." "Oh am I? ... I'd better change one of them to getting trampled by a rampaging hippogriff." Made even better by the fact that Professor Trewlaney loved it.
Made even better, when you realize that all of his made-up predictions come true (in a matter of speaking) during the course of the book.
Harry never got trampled by a rampaging hippogriff, though.
However, he did almost drown twice. Guess you don't get any points for redos.
Hilariously so, he would be more or less right a year ago.
Dumbledore meets Rita Skeeter:
Skeeter: How are you? I hope you saw my piece over the summer about the International Confederation of Wizards' Conference?
Dumbledore: Enchantingly nasty. I particularly enjoyed your description of me as an obsolete dingbat.
Another case, where Harry and Dumbledore are trying to cheer up Hagrid after Skeeter writes about his being half-giant:
Harry: Really, you shouldn't worry about what that Skeeter cow- er, sorry Professor.
Dumbledore: I have gone temporarily deaf and have no idea what you just said, Harry.
Harry: Erm... right.
And soon after:
Dumbledore: My own brother, Aberforth, was prosecuted for practicing inappropriate charms on a goat. It was all over the papers, but did Aberforth hide? No, he did not! He held his head high and went about his business as usual! Of course, I'm not entirely sure he can read, so that may not have been bravery...
Dumbledore: Really, Hagrid, if you are holding out for universal popularity, I'm afraid you will be in this cabin for a very long time.
The old wizard Archie, who, attempting to pass as a Muggle, wears Muggle clothing... which, unfortunately, happens to be a nightgown.
Ministry wizard: Muggle women wear them, Archie, not the men, they wear these.
Archie: I'm not putting them on. I like a healthy breeze 'round my privates, thanks.
Absolutely hilarious as read by Stephen Fry in the audio books.
The humor is even more heightened with the fact that Hermione of all people promptly gets the giggles and has to leave the line until Archie is gone.
When Harry meets Percy at the Yule Ball and Percy mentions how he's been promoted "As if he were announcing his election as supreme ruler of the universe."
Earlier the narrator mentions that Snape was as likely to just let his students play (due to the Yule Ball approaching) as nominating Harry his only heir.
After Fred mentions the Yule Ball:
Ron: Who're you going with, then? Fred: Angelina. Ron: What? You've already asked her? Fred: Good point. (calling across the room) Oi! Angelina! Angelina: What? Fred: Want to come to the ball with me? Angelina: All right, then. Fred:(to Harry and Ron) There you go, piece of cake.
Especially funny compared to Harry's agony over asking Cho.
Filch believes that he has finally caught Peeves stealing from a student, and would sure enough have him out of the castle when he presented Dumbledore with Harry's champion egg that he had just dropped while getting stuck underneath the invisibility cloak. Unfortunately for Filch, Moody arrives and takes it away.
"No!" said Filch, clutching the egg as if it were his first-born son.
"[Harry, Ron and Hermione] went into Gladrags Wizardwear to buy a present for Dobby, where they had fun selecting the most lurid socks they could find, including a pair patterned with flashing gold and silver stars, and another that screamed loudly when they became too smelly."
Molly Weasley's letter to the Dursleys, absolutely smothered in stamps. ("...Except for a square inch on the front, into which Mrs. Weasley had squeezed the Dursleys' address in minute writing.")
The Dursleys meet the Weasleys. The whole scene.
Fred and George seeing Dudley, and "breaking into identical evil grins."
The whole Skrewt subplot, especially taking them for walks, and when Hagrid is described ordering the class to come in for extra lessons to observe them "with the air of Father Christmas pulling an extra large toy out of his sack. "
Ron's new miniature owl, Pigwidgen, features in a number of these, the most notable being when he tries to deliver a letter in the middle of the entrance hall. A bunch of girls coo over him, until Ron grabs him in his fist, which he then shakes at the girls as he tells them to beat it. Pigwidgen is delighted the entire time.
Trelawny gives a long-winded speech about Harry's attributes that point to his birth date and guesses he was born midwinter. Harry corrects her that he was born in July. Doubly funny given that as an international celebrity, Harry's birthday would be a matter of public knowledge.
Later Trelawny rather heavily implies Harry will die a sudden violent death. Harry's reaction? Yawning as widely and obviously as he can.
Divination is a treasure trove of these. When tasked to look up the position of the stars at the moment of their birth, we get this interaction with Harry and Ron.
Harry: I've got two Neptunes here. That can't be right, can it?"
Ron: (Imitating Trelawny's mystical speech) "Ah young Harry, when two Neptunes appear in the sky, it is a sure sign that a midget in glasses is being born."
Don't forget this gem:
Lavender Brown: Oh Professor, look! I think I've got an unaspected planet! Oooh, which one's that, Professor?
Trelawney: It is Uranus, my dear.
Ron: Can I have a look at Uranus too, Lavender?
It's then followed up with this:
Most unfortunately Professor Trelawney heard him, and it was perhaps for that reason that she gave them so much homework.
After the Quidditch World Cup is over, the Bulgarian Minister reveals that he can in fact speak English, and was just having fun watching Fudge attempt to mime everything the entire day.
Frank Bryce thinking Voldemort and Wormtail are criminals/spies and the wizarding words they use are code. Imagine some guy in an Al Capone accent saying "Quidditch", "Muggles", and "Ministry of Magic" without skipping a beat.
Hermione's reaction when she is greeted by Dumbledore at Hagrid's door after she pounded at it and demanded to be let in.
Neville Longbottom performing a series of impressive gymnastics while under the influence of the Imperius Curse.
Crouch calling Percy, who hero worships him, "Weatherby", much to everyone's amusement.
Percy: It's classified information, until such time as the Ministry decides to release it. Mr. Crouch was quite right not to disclose it.
Fred: Oh shut up Weatherby.
From the film:
Ron complaining about Krum: "Ruddy pumpkinhead!"
Also Ron coming in pale and shaking and looking like he's about to faint because 'He just asked Fleur Delacour out."
"Actually, he sort of screamed at her."
And this exchange is especially hilarious because of Emma Watson's delivery.
Dumbledore pausing midway through prepping the champions to ask why Hermione is in the tent. She sheepishly excuses herself and leaves.
The brief shot of Cho Chang and Cedric Diggory dancing at the Yule Ball is utterly Hilarious in Hindsight. Robert Pattinson has this uber-serious look on his face that makes him look exactly like Edward Cullen.
When Ron is chosen by Professor McGonagall to demonstrate with her at the 'dancing lesson', Harry turns back to Fred and George...
Harry: "You're never going to let him forget this are you?"
Fred and George: (They look to each other, look back to Harry and shake their heads) Never.
Voldemort bum-rushing Harry during their duel at the climax.
It was said before, but the Amazing Bouncing Ferret scene is absolutely hilarious. Mainly for the fact that Moody ends up shoving Malfoy into Crabbe's trousers. (And Goyle reaches into the front of said trousers and fumbles around in there for several momnets.) As well as when Professor McGonagall arrives on the scene:
Professor McGonagall: Professor Moody! What are you doing?!