There are so many great ones! I think my favorito! is Ginny's "Anything's possible is you've get enough nerve" because it inspires me everyday. I also amor Ron's "Are tu a witch o not?" from the first book. Funny how they were both cut from the films, isn't it?
Mine is probably that good old Sirius Black quote - 'We all have light and dark inside us. It's the part we choose to act on - that's who we really are.' I think about that quote all the time. It's pretty similar to the batman one, 'It doesn't matter who tu are underneath; it's what tu do that counts'. I wonder who came up with it first.