As for me, it's Princess Mononoke , because it shows a really beautiful world with on one hand animals and nature and on the other hand humans and it presents a poetic story about that conflict. What about you ?
I must say: "Howl's moving castle". Helps me to bring out the children inside me. You know, forget about everything for two hours.
Princess Mononoke it's a great movie too, is just that it reminds me how sad i feel when i see living beings suffering or being destroyed, it's sad to watch all that beautiful things being corrupted (in the movie and reality).
Miyasaki has many great movies. It's always a pleasure to watch them.