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.
Princess Mononoke for me as well, but Nausicäa has also a special place in my heart.
The reason for that is that in my childhood, there was a mystical animated movie in the video store next to my home that I didn't quite understand but which captivated me like no other, which is why I rented it over and over again. But one day, the store had gotten rid of it, which made me quite sad. Over time, I could only recall some of the images and the name, which was "Star-Warriors" (although the story wasn't set in space). This was all pre-internet.
Fast forward years later, I'm an adult and I have already watched Princess Mononoke and Chihiro.
When I came upon an older movie by Studio Ghibli called Nausicäa, of course I watched it immediately. There was also something about the cover image which was vaguely familiar. Imagine my surprise when that movie turned out to be the magical movie from my childhood, but longer and better.
As it turned out, the reason I didn't understand it wasn't that the movie was to complicated for my age, but because I watched a Western version that had been butchered by cutting out over twenty minutes of the story, which was most of the scenes dealing with concepts such as pacifism and the conservation of nature, also changing the names of the characters, so Nausicäa was now "princess Sandra" and the Ohmus were called "Gorgon monsters".
Ah yes, Warriors of the Wind. That was the version that made me fall in love with Studio Ghibli, and after finally watching Nausicaä in my twenties I became a lifelong fan.