Howl's Moving Castle is definitely my favorite. Whenever I watch it, I don't feel like I just watched a 1.5 hour movie. I feel like I read a ~20 hour novel. The world and characters are so rich with detail--there's a lot that the movie barely scratches the surface of and just leaves unsaid, and I love it. I feel like the more thoroughly you explore a world and explain EVERYTHING about EVERY SINGLE land and EVERY SINGLE city, the smaller it becomes. It also helps that they visit many places, again not seeing everything these places have to offer, but just enough to know they have their own things going on.
And so much happens in that short amount of time, as well. Sophie gets old and she gets young, there's the stuff with the Witch of the Waste, and Madame Salaman, and the move, and going to the past, and the flower garden, and the cleaning, and the lake, and then back to the flower shop...so much happens, yet none of it seems rushed. The pacing is amazing.
Spirited Away is a pretty close second, just because it's such a sweet and unique story. Like Howl's Moving Castle, a lot happens and a lot of interesting places are visited but the pacing gives each of these things all the time they need. The reason I like Howl better I guess is because I like the world better. Howl's Moving Castle just feels more detached and fanciful to me, I guess because in Spirited I'm aware that a lot of what's going on is based on Japanese folklore. I also love every other film of his I've seen--there isn't one I'd say I dislike. I haven't seen Porco Rosso, Pom Poko, Grave of Fireflies, or Castle of Caligostro, but not because I don't want to. I simply haven't had the chance yet. And I'm REALLY hoping his newest film gets at least a few showings in my area when it hits the states. I didn't miss Arietty, and I don't plan on missing it either. I realize neither of those are directed by Miyazaki himself, but he still had to do with their writing so I'm still excited.