I've only watched two: Spirited Away and Howl's Moving Castle. I don't love any of them to bits, but they're both very well-animated, detailed and sweet movies. In chosing between the two, Spirited Away would be my favourite, since I absolutely adore Dianna Wynne-Jones' novel, and frankly, the movie was so loosely based on her book that it kind of took away the fun. Plus, the storyline was terribly confusing, despite me having read the book. So yeah, Spirited Away it is.
But I am absolutely dying to watch Grave of the Fireflies and Mononoke-hime: problem is, they're nowhere to be rent and I can't find them for download anywhere. If anyone could send me a pm with where I can find these two movies I'd be so, so happy. *puppy eyes*