As far as I'm concerned,
Princess Mononoke is not only a fine movie in its own right, it's also the best, most balanced piece of entertainement which deals with environmental issues.
In the movie, nature is depicted as something precious, but its denizens are not necessarily benevolent, the main "villain" is an equal rights activists and in the end, although there's a resolution of sorts, there is no reset button and we're meant to understand that nothing will ever be the same anymore. It's truly impressive, especially when you contrast it with Miyazaki's "Nausicaä", which deals with the same themes in a much more naive way.