The first line from SS that made me laugh out loud: "(Uncle Vernon) hummed 'Tiptoe Through the Tulips' as he worked, and jumped at small noises."
My favorite line from DH was near the end during the King's Cross chapter, when Harry asks Dumbledore if he's dead, and Dumbledore replies, "On the whole, I think not" or something like that. (I lent the book to my mom, so I can't look up the exact quote.) I like this line, first, because it's classic Dumbledore, and because it finally answered the big question...Will Harry live?