I think these are all worth seeing if you can catch them:
Spirited Away
In my all-time top ten best films, where it sits uneasily among all the death and destruction, this is something like a Japanese Alice in Wonderland, and I love every second of it.
Princess Mononoke
A morally complex fable about industrialisation and civilisation. Packs a massive spiritual punch at the end.
Howl's Moving Castle
An adaptation of a Diana Wynne-Jones book. The most recent, and most diluted, Ghibli film. Four stars rather than five, but has lots of good stuff in it.
Grave of the Fireflies
A teenage boy and his baby sister try to survive after their mother's death in a bombing raid during the 2nd World War. The saddest film I have ever seen. I sobbed for the final hour. I'm distressed remembering it. A very good film, but heart-wringing.
Are there any others you would recommend?