For me, it's Mamoru Oshii's masterpiece Tenshi no Tamago (1985). Gorgeous animation and a very atmospheric story with barely any dialogue, will leave you scratching your head.
For something more modern and accessible, I loved Eiga Daisuki Pompo-san / Pompo the Cinephile (2021), a love letter to cinema (specifically Hollywood-style cinema, from a japanese perspective); another great one from recent years is The Tale of the Princess Kaguya / Kaguya-hime no Monogatari (2013), but maybe choosing a Ghibli film is cheating?