I think that for me it's "Spirited away" - http://www.youtube.com/watch?v=jiEzl7SEtkI

The story of how the heroine matures in front of our eyes as the weight of the world is placed upon her shoulders was really moving to me. Coupled with beautiful hand-drawn animation and an overall sense of gentleness and wander, it's been an real favorite for me which I enjoy watching time over time.