my fav has to be

You're a prefect? Oh Ronnie! That's everyone in the family!" [Molly Weasley]
'What are Fred and I? Next door neighbours?"-George

and can't forget this one:

George: Someone may slip dragon dung in it again, eh Percy?
Percy: That was a sample of fertiliser from Norway! It was nothing personal!
Fred (whispers): It was. We sent it.

:fox: