Lockhart: Why not ask Professor Snape to show you how to whip up a love potion!
Snape was looking as if the first person to ask him for a love potion would be force-fed poison.
Made funnier by the fact that Love Potions are banned at Hogwarts.
"Make way for the heir of Slytherin, seriously evil wizard coming through!"
"Soon the air was thick with flying gnomes."
"... the gnome, sensing weakness, sank its razor-sharp teeth into Harry's hand. He had a hard job shaking it off until- 'Wow, Harry! That must've been fifty feet!'"
Just the entire way the Weasleys are so matter-of-fact about the de-Gnoming. Harry is shocked at the process (yank an incredibly ugly creature out of the bushes, spin them over your head to get them good and dizzy, and fling them far enough away that they can't find their way back), and to the Weasley kids, it's just like weeding the yard.
The Weasleys and Harry watching the gnomes stagger off, while one of them comments that they'll just show up again because they love living there. Bonus for the casual mention later, that when Harry glances out of Ron's bedroom window at the end of the chapter, the gnomes are already filing back into the yard.
After Snape fails to get Harry and Ron expelled, he's described as looking "as though Christmas had been canceled."
Becomes Actor Allusion when one remembers Alan Rickman's earlier turn as the Sheriff of Nottingham:
"Cancel the kitchen scraps for the widows and orphans, no more merciful beheadings...and CALL OFF CHRISTMAS!"
*Referring to getting Moste Potente Potions via telling a teacher they're "just interested in the theory" of Polyjuice Potion*"Oh, come on, no teacher's gonna fall for that!" said Ron, "they'd have to be really thick..." [end chapter] [start of next chapter] Since the disastrous episode with the pixies, Professor Lockhart had not brought live creatures to class...
I can't remember if it was stated in the book, but where did the basilisk come from?
Salazar Slytherin hatched it a thousand years ago. It's been waiting all this time for the Heir of Slytherin to come and say "Wanna play fetch? Go get the mudblood! Go get it! Good giant snake thing! Atta boy!" *
It's actually a girl.
The Howler. All of it.
"Potter, can I have your picture, Potter? Can I have your autograph? Can I lick your shoes, please, Potter?"
When Ron grabs Hermione's timetable out of her hands and says, "Why have you outlined all Lockhart's lessons in little hearts?!"
Also when Madame Pince tries to take the slip with Lockhart's signature on it away from Hermione and she asks, "I was wondering if I could keep it?" and Ron says, "We'll get you another autograph. Lockhart will sign anything if it stands still long enough."
"His eyes are as green as a fresh pickled toad..."
From the film:
Funniest line in the Harry Potter movies which wasn't in the books: After Harry and Ron are sent into the Forbidden Forest to "follow the spiders", Ron, who had a previously established fear of spiders, quips "Why spiders? Why couldn't it be 'follow the butterflies'?"
Made even funnier by the director taking advantage of Rupert Grint's voice change hitting mid-movie. Yeah, that voice-cracking wasn't completely intentional.
When Mrs. Weasley tells Mr. Weasley about how Ron, Fred and George stole the flying car to rescue Harry from the Dursley's:
Mr.Weasley: Did you really? How'd it go? (nudged by outraged Mrs. Weasly) err, I mean, that was very wrong of you indeed, boys, very wrong of you.
Exchange between Malfoy and Harry posing as Crabbe: "Why are you wearing glasses?" "Uh... reading." "Reading? (Beat) I didn't know you could read."
Topped off by the look on Tom Felton's face, like the thought of Crabbe being able to read is incredibly impressive. (Which, given how stupid he apparently is, is pretty likely)
Especially since it was later stated that Tom Felton ad-libbed the line...aged what, 11? Awesome.
The Howler is even more hilarious in the movie than in the book. It actually stops shouting to compliment Ginny on getting into Gryffindor, turns itself into a face to speak, blows a raspberry at Ron, then tears itself to pieces. And the expression on Rupert Grint's face is hysterical.
Another hilarious moment not in the book - Harry and Ron are flying the car and wondering where the train is, and then they get this highly amusing horrified look on their faces as they realize the Hogwarts Express is closing in fast from behind them.
To top it off, Hedwig's the one that notices first, and gets as close to a live-action Eye Pop as possible.