Well, there are way too many but I'm going to stick to these -
Ones by Fred and George
"Make way for the heir of Slytherin, seriously evil wizard coming through..."
"Cheers," whispered George, wiping tears of laughter from his face.
"Oh, I hope she tries Vanishing them next...they multiply by ten every time you try..."
"E for 'Exceeds Expectations.' And I've always thought Fred and I should've got E in everything, because we exceeded expectations just by turning up for the exams."
"Fred you're next " the plump woman said .. "I'm not Fred I'm George " said the boy ."honestly woman, call yourself our mother? can't you tell I'm George" .."sorry George dear" "only joking I'm Fred
"You're a prefect? Oh Ronnie! That's everyone in the family!" [Molly Weasley]
"What are Fred and I? Next door neighbors?"
"Time is Galleons, little brother."
"You can't give a Dementor the old one-two!"
"Wow, I wonder what it'd be like to have a difficult life?"
"Pity you can't attach an extra arm to yours (Malfoy's broom), Malfoy. Then it could catch the Snitch for you."
"I don't go looking for trouble. Trouble usually finds me."
"There's no need to call me sir Professor."
"It's just hard," Harry said finally, in a low voice, "to realize he won't write me again." by Harry (when Sirius died)
"SNAPE KILLED DUMBLEDORE"
"Hearing voices no one else can hear isn't a good sign, even in the wizarding world."
"Sunshine, daisies, butter mellow,
Turn this stupid, fat rat yellow."
"And what in the name of Merlin's most baggy Y Fronts was that about?"
"Well, I don't know how to break this to you, but I think they might have noticed we broke into Gringotts."
"It's the unknown we fear when we look upon death and darkness, nothing more."
"It does not do to dwell on dreams and forget to live."
"Fear of a name increases fear of a thing itself."
"Your mother died to save you. If there is one thing Voldemort cannot understand, it is love."
"It is our choices that show what we truly are, far more than our abilities."
"To the well organized mind death is but the next adventure"
Some miscellaneous ones:
"Wit beyond measure is man's greatest treasure" by Luna.
"The mind is not a book, to be opened at will and examined at leisure." by Snape.
"You should have died! Died rather than betray your friends, as we would have done for you!" by Sirius.
"You don't know what I'm capable of, you don't know what I've done!" by Draco.
"NOT MY DAUGHTER YOU *****" by Molly.
I could go on..