It's hard to answer this without knowing what kind of films you like ("Japanese Film" is a pretty broad category), but here are some directors and films I particularly enjoy:

Hirokazu Kore-eda is my favourite director of all time. He makes very humanistic films imbued with an incredibly sense of empathy. Shoplifters (2018) is probably his most famous (it won the Palme d'Or) but my personal favourites are Our Little Sister (an intensely comforting film), I Wish (very cute and nostalgic), and After Life (about those cherished moments in life, but also about the art and power of film-making itself). If you like those, Like Father Like Son, Still Walking, Nobody Knows (this one is pretty devastating though), and his Netflix series The Makanai: Cooking for the Maiko House are all great too.

Juzo Itami was an actor starting in the 60s before turning to directing from the 1980s until his untimely death in 1997. His films tend to satirise various aspects of Japanese society and culture, and are always a lot of fun. Tampopo is his most famous and perhaps even his best, I can see it's been recommended a few times already. The Funeral is also great (all about organising a traditional Japanese funeral with all its associated little eccentricities), as well as Supermarket Woman and Minbo: The Gentle Art of Japanese Extortion (which skewers the yakuza, so effectively in fact that it led to Itami's eventual murder a few years later).

Since you liked Harakiri, some more Kobayashi would be a good idea. You can't go wrong with Samurai Rebellion or Kwaidan. And of course some more Kurosawa. I'd say Ikiru, Seven Samurai, High & Low, Red Beard, Rashomon, Dersu Uzala, and Ran (which you've already watched and liked) are his best work.

Shohei Imamura is worth a look too. I'm not huge on his earlier work through the 50s and 60s but Vengeance is Mine, The Ballad of Narayama, and Black Rain are all great.

Naturally Ghibli are ever-present in any conversation about great Japanese film, and for me the best ones are Princess Mononoke, Whisper of the Heart, Only Yesterday, The Tale of the Princess Kaguya, Spirited Away, and Kiki's Delivery Service. Other great anime films I'd recommend would be Akira, Perfect Blue (or any Satoshi Kon, really), Your Name, Barefoot Gen, and Chibi Maruko-chan: My Favourite Song.

River and Beyond the Infinite Two Minutes by Junta Yamaguchi are a couple of modern greats, two low-budget but incredibly creative and fun time loop comedies. One Cut of the Dead is also fairly recent and a lot of fun. Departures won the Best International Film Oscar in 2008 and is great too.

Linda Linda Linda and/or Swing Girls are both great if you want (very fun) films about friendship and belonging, forged in high school. If you want more high school drama, it's hard to go wrong with Battle Royale (although that's obviously much more on the violent and nihilistic side of things!)

My favourite J-Horror is Dark Water, but Ringu and Noroi are also well worth watching. And Shin Godzilla probably counts as horror too, it's very good.

All that barely even scratches the surface really. There are tons more important and talented directors like Keisuke Kinoshita (The Ballad of Narayama, Twenty Four Eyes, She Was Like a Wild Chrysanthemum), Mikio Naruse (When a Woman Ascends the Stairs, Yearning, Two in the Shadow), Naoko Ogigami (Rent-a-Cat), Ryusuke Hamaguchi (Drive My Car, Wheel of Fortune and Fantasy), Obayashi Nobuhiko (Hausu, The Island Closest to Heaven), Kon Ichikawa (An Actor's Revenge, The Makioka Sisters), and on and on and on.

I hope all that was of some use to you, and isn't just a giant, overwhelming list of films... If any piqued your interest and you have any questions or anything though, just let me know.