Weird nobody mentioned Flower Travellin' Band or DJ Krush yet.
In between the Chris Haikus avatar and the recommendations for literally precisely the two artists I was thinking about throughout reading the thread, you just became my new favorite music forumite.
The Flower Travellin' Band are fucking epic, and DJ Krush is sickness beyond sick.
Ki-Oku and
Zen are the only two I've even been able to pick up and they are not just the cat's pajama's but the dog's whole wardrobe.
I have to talk about Michio Kurihara, Rururu and sgt. though ... Japan's good post-rock doesn't end with Mono and té.
Green Milk From Planet Orange is awesome as well, and Mainliner is heavy as balls. Eternal Elysium can lay down some absolutely righteous riffage that is basically as good as Sabbath if you get past the fact that they sound like a tribute band every so often.
But it really does come down to Acid Mothers Temple, Boris (in all its incarnations), and Mono. These three bands are enough to make Japan better than anyone.