In order:
Princess Mononoke
My Neighbor Totoro
Nausicaä of the Valley of the Wind
Spirited Away
Howl's Moving Castle
Kiki's Delivery Service
Laputa - Castle in the Sky
Porco Rosso
To put this list in perspective: I even like Porco Rosso more than 95% of (modern) western animation movies.
Whisper of the Heart and The Cat Returns were good as well, but not as good as Myiazaki's own movies.
I still plan to buy and watch Pom Poko, My Neighbors the Yamadas, Grave of the Fireflies and Only Yesterday some day.
Ponyo on the Cliff by the Sea was not even in cinemas here yet (will be in September), but as I loved Kiki and Totoro, Ponyo is another definite buy once it comes out on DVD (I don't go to the cinema anymore).
The only Ghibli movie that I didn't like very much was Tales from Earthsea. It was as beautiful as expected, but at the same time extremely boring to me because of the weak screenplay.
