Spirited away is my favorite, closely followed by princess mononoke, grave of the fireflies, howl's moving castle, kiki's delivery service, my neighbor totoro, and whisper of the heart.
Spirited Away is one of the most creative and emotional movies ever imo. It has everything necessary for a great animated movie.
Grave of the Fireflies is really powerful emotionally. That movie had me down for like a week. I haven't watched it since, I'm not emotionally prepared yet