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.
Very hard to pick one but The Wind Rises has a special place in my heart (tbf most Ghibli movies do). First of all, I watched it when I was very young and forgot about it so it was like rediscovering something I've almost forgotten. Second, I'm in and engineering program and know a bit about planes and such so I could relate to the MC. Third, I love the more adult feel of it and the Shakespearian romance. Lastly, it does all the usual Ghibli stuff great like animation, ost and wholesomeness.
I'm in a technology center half the day and my highschool the other half. Its more of an introduction to different engineering subjects along with calculus and physics. Aerospace was one of the main courses we took last year which was the same time I watched The Wind Rises.