For me it would be Princess Mononoke, which is also one of my favorite films, period. I really appreciate the maturity and nuance it brings to it's environmental themes, which have been done in many other films, but not as well.
not exactly ghibli but i love nausicaä of the valley of the wind. one of my favorite movies in general. it has my favorite take on the environmentalist thread that so many miyazaki movies have, and the world building is a favorite of mine in anime, with so much more to offer in the manga. not to mention out of all the miyazaki flying contraptions, nausicaä's glider is definitely the coolest.