No one can say it quite like J.K. Rowling. Rather than posting the text for the entire series (I'm pretty sure that wouldn't be legal), I have picked a few of my favorite Harry Potter quotes and copied them here for my enjoyment, mostly.
Harry Potter and the Sorcerer's Stone
"I hope you're pleased with yourselves. We could all have been killed—or worse, expelled." —Hermione
"Yeah, and lucky Harry doesn't lose his head in a crisis—'there's no wood,'
honestly." —Ron (This part is even more awesome when you read the 7th book and Ron laments that they didn't have Crookshanks when they were going to spy on Voldemort in the Shrieking Shack and Hermione says, "Are you a wizard or not?!"
"Are you really Harry Potter?" —Ron
"Mars is bright tonight." The centaurs
"Where's the canon?" —Dudley
"Nitwit! Blubber! Oddment! Tweak!" —Dumbledore
"I suppose she thinks you don't forget your name. But we're not stupid—we know we're called Gred and Forge." —George
"Scars can come in handy. I have one myself above my left knee that is a perfect map of the London Underground." —Dumbledore
Harry Potter and the Chamber of Secrets
"Lockhart will sign anything if it stands still long enough." —Ron
"Watch out for that tree!" —Harry
"It is our choices, Harry, that show what we really are, far more than our abilities." —Dumbledore
"You know what, Harry? If he doesn't stop trying to save your life, he's going to kill you." —Ron
"I want more bacon." —Dudley
"I hope Ron's not in a girl's toilet." —Percy
Harry Potter and the Prisoner of Azkaban
"Well, honestly . . . 'the fates have informed her' . . . who sets the exam? She does! What an amazing prediction!" —Hermione
"Tell me, which of you will be dying this year?" —Professor McGonagall
"Pity you can't attach extra arms to yours, Malfoy. Then it could catch the snitch for you." —Harry
"Mr. Padfoot would like to register his astonishment that an idiot like that ever became a professor." —Padfoot
Harry Potter and the Goblet of Fire
"Ecletic you say? With a plug? Gracious, I must see that." —Mr. Weasley
"CONSTANT VIGILANCE!" —Mad-Eye Moody
"You coming to this ball thing on Christmas day, Hagrid?" —Ron
"I have gone temporarily deaf and haven't any idea what you said, Harry." —Dumbledore
"I killed my father." —Barty Crouch Jr.
"Don't be stupid . . . it's a flying house!" Dennis Creevey
"What are you doing here?" Rod and Fred
"I want to fix that in my memory forever. Draco Malfoy, the amazing bouncing ferret." —Ron
"Did I tell you I've invented a broomstick that'll reach Jupiter?" —Ron
"Er, is this the new stand on elf rights? You're going to make yourself puke instead?" —Ron
"If the Hogwarts Express crashed tomorrow, and George and I died, how would you feel to know that the last thing we ever heard from you was an unfounded accusation?" —Fred
Harry Potter and the Order of the Phoenix
"Don't put your wand there, boy! What if it ignited? Better wizards that you have lost buttocks, you know!" —Mad-Eye Moody
"There's been a mistake. No one in their right mind would make Ron a prefect." —Fred
"Special discounts to Hogwarts students who swear they're going to use our products to get rid of this old bat." —George
"Just because you have an emotional range of a teaspoon doesn't mean we all have." —Hermione
"The mind is not a book, to be opened at will and examined at leisure. . . . The mind is a complex and many-layered thing." —Snape
"I didn't think there was anything in the universe more important that homework." —Ron
"We believe the dementors are currently taking direction from Lord—Thingy." —Fudge
"The world isn't split into good people and Death Eaters." —Sirius
"I expect what you're not aware of would fill several books, Dursley." —Mad-Eye Moody
"Size is no guarantee of power." —George
"You know Minister, I disagree with Dumbledore on many counts, but you cannot deny he's got style." —Phineas Nigellus
"Ow kunnit nofe skusin danger ifzaf?" —Ron
"Wow, I wonder what it'd be like to have a difficult life?" —Harry
"Well, that clears it up. It would have been really annoying if you hadn't explained yourself properly." —Ron
"You don't want to bottle your anger up like that Harry, let it all out. There might be a couple people fifty miles away who didn't hear you." —Fred
"What are Fred and I? Next-door neighbors?" —George
"The thing about growing up with Fred and George, is that you sort of start thinking anything's possible if you've got enough nerve." —Ginny
"I could have gotten rid of the sparklers myself, of course, but I wasn't sure whether I had the authority . . ." —Professor Flitwick
Harry Potter and the Half-Blood Prince
"It was a lucky day for the Weasleys when Ron decided to sit in your compartment on the Hogwarts Express, Harry." —Mr. Weasley
"There is nothing to be feared from a body, Harry, any more than there is anything to be feared from darkness." —Dumbledore
"Your father and I were made for each other, what was the point in waiting?" —Mrs. Weasley
"There's no way they'd let me be a Death Eater!" —Ron
"I'm tall." —Ron
"What have you been doing to that book, you depraved boy?" —Madam Pince
"That Harry Potter's got more backbone than the whole Ministry of Magic put together!" —Neville's gran
"It's high time your grandmother learned to be proud of the grandson she's got, rather than the one she thinks she ought to have." —Professor McGonagall
"I've learned not to expect too much from boys." —Moaning Myrtle
"Excuse me, are you the imprint of a departed soul?" —Ron
"I am good-looking enough for the both of us!" —Fleur
"Once again, you show the sensitivity of a blunt axe." —Nearly Headless Nick
"She didn't see that, did she?" —Ron
"There's no need to call me
sir, professor." –Harry
"I am not worried, Harry. I am with you." —Dumbledore
"Nobody's ever asked me to go to a party before, as a friend. Is that why you dyed your eyebrow, for the party? Should I do mine, too?" –Luna
Harry Potter and the Deathly Hallows
"I don't think you're a waste of space." Dudley
"Vot is the point of being an international Quidditch player if all of the good-looking girls are taken?" —Krum
"The fact remains he can move faster than Severus Snape confronted with shampoo when he wants to." —Fred or George
"Well, I don't know how to break this to you, but I think they
might have noticed we broke into Gringotts." —Ron
"Merlin's pants!" —Hermione
"And what in the name of Merlin's most baggy Y Fronts was that about?" —Ron
"Perhaps just one more, Master Harry, for luck?" —Kreacher
"Daddy, look—one of the gnomes actually bit me!" —Luna
"Imagine losing fingernails, Harry! That really puts our suffering into perspective, doesn't it?" —Hermione
"OI! There's a war going on here!" —Harry
"What's that, an illness?" —Ron
"If you should feel any burgeoning talent today—perhaps an unexpected urge to sing opera or to declaim the Mermish—do not repress it!" —Xenophilius Lovegood
"If there was a wizard of whom I would believe that they did not seek personal gain, it would be you, Harry Potter." —Griphook
"A brutal triple murder by the bridegroom's mother might put a bit of a damper on the wedding." —Ron
"Brains like that, you could be a Death Eater, son." —Aberforth
"You know how to drive, I take it?" —Dedalus Diggle
"We teachers are rather good at magic, you know." —Professor McGonagall
"IF WE DIE FOR THEM, I'LL KILL YOU HARRY!" —Ron
"Imagine if something went wrong and we were stuck as specky, scrawny gits forever." —Fred
"Can’t you even tell us apart when we’re Harry?" —George
"You look much tastier than Crabbe and Goyle, Harry." —Hermione
"It's me. I'm extremely famous." —Ron