There are so many great lines to chose from all the books....
There is no good or evil: only power and those too weak to seek it.
Harry Potter and the Sorcerer's Stone
Spoken by Quirinus Quirrell.
It takes a great deal of bravery to stand up to our enemies, but just as much to stand up to our friends.
Harry Potter and the Sorcerer's Stone
Spoken by Albus Dumbledore.
But the best quotes are from book 5 of corase. :)
Page 383(This is my favorite quote from all of these I think)
"Harry, however, had never been less interested in Quidditch; he was rapidly becoming obsessed with Draco Malfoy."
Page 391, Ron's conversation with Harry
Ron, "You're getting a bit obsessed with Malfoy, Harry. I mean, thinking about missing a match just to follow him..."
Page 429, Harry attempting to enter the Room of Requirement
Harry, "I need to see the place where Malfoy keeps coming secretly"
Harry tried every variation of 'I need to see what Draco Malfoy is doing inside you.'(He is jealous of a room LOL)