My favorite film is Mimi wo sumaseba (Whisper of the heart). But actually all of them are beautiful. Nausicaa of the Valley of the Wind means so much to me, in so many ways. The fact that Ghibli's work don't try to teach us a lesson but they allow us to think about life.
I will always be grateful for Miyazaki sensei and Takahata sensei (they are my favorite storytellers and filmmakers). They are so special and I think that is because they always stay true to themselves. :D