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.
Language is English but Japanese titles and short comments eg. “かわいい!” are allowed. This is because my Japanese is shit and thus I cannot moderate it effectively.
Porco every time for me, feels like a mini holiday every time I watch it, but Totoro would be a close second.