The conclusion, when Peeves chased Umbridge off by hitting her with a walking stick and a sock filled with chalk. Professor McGonagall's reaction? "I would chase after her myself, but Peeves borrowed my stick." Made funnier when listening to the book on tape, where the reader's McGonagall voice seems to be hinting that Peeves didn't "steal" the stick so much as "asked politely for it and detailed the exact reason why he needed it."
Pretty much any time McGonagall and Umbridge interacted in Order of the Phoenix was this, often with an added Crowning Moment Of Awesome for McGonagall.
Molly: I don't believe it! Oh, Ron, how wonderful! A prefect! That's everyone in the family!
George: What are Fred and I, next-door neighbors?
And Alastor "Mad-Eye" Moody has his own perspective about Ron's said promotion: "Authority figures always attract trouble, but I suppose Dumbledore thinks you can withstand most major jinxes or he wouldn't have appointed you…"
Molly's reaction to Arthur's stitches: "WHAT DO YOU MEAN, THAT'S THE GENERAL IDEA?!"
After Dudley is attacked by a dementor at the beginning:
Vernon: Fought 'em off, did you, son? Gave 'em the old one-two, did you?
Harry: You can't give a dementor the old one-two!
Harry's not recorded as facepalming during that line, but it's not hard at all to imagine it.
When talking about the dementor attack, Vernon demands to know what the hell they are. Petunia promptly answers "They guard the wizard prison, Azkaban" and then slaps her hands over her mouth in horror.
Umbridge's evaluations of the teachers could be both funny and painful. Snape's was particularly memorable:
Umbridge: You applied first for the Defense against the Dark Arts post, I believe.
Umbridge: But you were unsuccessful?
Snape: (classic deadpan) Obviously.
Then there's Ron's reaction to hearing the evaluations are going to take place. Everyone else is frightened for their favorite teachers, but Ron...
Ron: I cannot wait to see McGonagall evaluated! Umbridge will never know what hit her!
Mad-Eye: Don' put your wand there, boy! What if it ignited? Better wizards than you have lost buttocks, you know!
Tonks: Who do you know who's lost a buttock?
When they sneak in to the Ministry of Magic to save Sirius, they all get name tags saying "Rescue Mission". It gets even better as it's ambiguous whether or not they put them on, so if you want you can picture them wearing name tags throughout the ensuing battle. Luna Lovegood was there. She almost certainly put hers on.
Two particularly hilarious dreams: Before Harry has his out-of-body experience in Nagini attacking Arthur, he dreams Cho is angry at him for luring her to the DA under false pretenses by promising her 150 Chocolate Frog Cards: "Cedric gave me loads of Chocolate Frog Cards, look!" Then she becomes Hermione who says he did promise Cho and suggests he should give her his Firebolt, but Umbridge has it and he just came to the DA to put up Christmas ornaments shaped like Dobby's head… The second dream involves him watching Neville and Professor Sprout waltzing in the Room of Requirement while McGonagall plays the bagpipes. This is not out of character for McGonagall either, as she is Scottish and, according to Pottermore, made her father's bagpipes play on their own as a baby!
A metaphor where Luna is described as staring at Ron "as if he were a mildly interesting television program.''
When Ron is freaking out before his first Quidditch match: "Ron was now staring into the dregs of milk at the bottom of his empty cereal bowl as though seriously considering attempting to drown himself in them." and "Harry and Ron pulled on their robes (Ron attempted to do his up back-to-front for several minutes before Alicia took pity on him and went to help."
After Fred and George's escape, Ron, Hermione, and Harry are all talking about it, and Ron is sure his mother is going to blame him for allowing it. Then Harry reveals that he was the one who gave them the money to start their joke shop, and we get this:
Ron: "But this is excellent! It's all your fault! Can I tell mum?"
Ron's comment upon dropping Divination (after one his exams, he had told the examiner in detail about the ugly man with a wart on his nose in his crystal ball, only to look up and realize he had been describing his examiner's reflection):
Ron: "And from now on I don't care if my tea-leaves spell 'Die, Ron, die!' I'm chucking them in the bin where they belong."
As well as Ron's comment about "Jupiter [getting] too friendly with Uranus" or something of the like. Ron says "Uranus"three times between GoF and OoTP, and it's lampshaded twice - in other words, there's no way Ron's choice of words here was an accident.
Harry's examination is pretty good too, including mixing up the lines on his (very old) examiner's palm and telling her she should have died the previous Tuesday.
Near the beginning we have Mrs. Figg, Harry's batty old Squib neighbor, batteringMundungus Fletcher with a bag full of canned cat food.
The aftermath of Harry's angsty tirade towards Ron and Hermione. He goes on and on about how no one understands him, only for Fred, George, and Ginny to pop into the room and casually and snarkily greet him with "Oh hey, Harry! Thought we heard you!"
Fred and George Apparating into Harry and Ron's bedroom to talk about the weapon Voldemort wants to steal, leading to this exchange:
After Umbridge used up Snape's Veritaserum (which actually turned out to be fake) and demands he make more to interrogate Harry again, Snape tells her it will take a month unless he wishes to poison Harry. Umbridge throws a tantrum, and puts Snape on probation on the spot.
The description of Filch, his teeth gnashing, "punting" students across the swamp that Fred and George left behind in the Great Hall. Of course Jo's talking about the boat, but to those not familiar with that specific term, it can paint...a rather different picture.
The scene in the Hogwarts Express when Ron was planning on what to do to punish Crabbe or Goyle.
Ron: I'll make Goyle do lines, it'll kill him, he hates writing. (He lowered his voice to Goyle's low grunt and, screwing up his face in a look of pained concentration, mimed writing in midair) I ... must ... not ... look ... like ... a ... baboon's ... backside.
Luna becomes "rather bored by the proceedings" while Umbridge is threatening to expel her and her friends.
Ginny imitating Umbridge's "Hem hem" cough...accurately. In fact, it sounded so realistic that people thought she was there with them!
It's a pretty dark scene overall, but the Marauders in the Snape's Worst Memory chapter are genuinely funny at least among each other.
Sirius: So what did you think of the fifth question, Moony?
Remus: Excellent one. Give us five signs that identify the werewolf?
James:(in a mocking tone) Do you think you missed any?
Remus: Let's see, one, he's sitting on my chair, two, he's wearing my clothes and three, his name is Remus Lupin.
(James and Sirius laugh)
Peter: I'm not sure I got all of it.
James: Wormtail, how thick can you get? You run around with a werewolf once a month.
For Severus, it's unending horror, but the Black Comedy of James torturing Snape as an excuse to hit on Lily is incredibly funny while being Dude, Not Funny! at the same time. Especially James' petulant reply when Lily blows him off.
James:(in a would-be Macho tone) What's with her?
Sirius:(like talking to a small child) Reading between the lines, mate, I'd say she thinks you are a tad conceited.
The description of Ron and Hermione's prefect duties in December (interspersed with Ron's complaints about them):
"They were called upon to supervise the decoration of the castle ('You try putting up tinsel when Peeves has got the other end and is trying to strangle you with it,' said Ron), to watch over first- and second-years spending their break-times inside because of the bitter cold ('And they're cheeky little snot-rags, you know, we definitely weren't that rude when we were in first year, said Ron) and to patrol the corridors in shifts with Argus Filch, who suspected that the holiday spirit might show itself in an outbreak of wizard duels ('He's got dung for brains, that one,' said Ron furiously)."
In an otherwise Tear Jerker scene, Dumbledore's passive non-reaction to Harry's Tantrum Throwing: "By all means continue destroying my possessions. I daresay I have too many."
Hermione's Christmas presents for Harry and Ron: homework planners that talk after something is written in them. It's never explained why she thought they'd appreciate her gift, but she might've just wanted the planners to remind them of schoolwork so she wouldn't have to.
"'Don't leave it till later, you big second-rater!' chided the book as Harry scribbled down Umbridge's homework. Hermione beamed at it. 'I think I'll go to bed,' said Harry, stuffing the homework planner back into his bag and making a mental note to drop it in the fire the first opportunity he got."
At one point, we see Umbridge evaluating Flitwick's music class. She ends up measuring him. The "Are you kidding me?" look on his face is priceless.
And after the twins' escape it gets a Call Back; Flitwick does a victory pose.
Crookshanks eating the Extendable Ear.
And Fred and George's reaction - "Bloody cat!"
When the trio meet Grawp, he picks up Hermione and she tries to order him to put her down. Emma Watson's tone of sheer terror pretending to be anger is priceless. Even more hilarious when it works and Grawp gives her a present of a set of bicycle handlebars.
When Umbridge is being carried off by centaurs we hear her "Let me gooooooooooooooooooooooooo!" echo off into the distance and it's hysterical.