I have been listening to 77Boadrums and Re-Bore vols. 1 and 2 for a week straight. When I'm tired of those, I put in Seadrum/House of Sun.
UA, a singer-songwriter, is more poppy, but still experimental. She has a lot of different styles - jazz, dub, dance, rock, pop. You won't hear her lumped in with every other cookie-cutter JPop artist. She's more comparable to Bjork. The project AJICO she was in was pretty straight-ahead grungy rock with a lot of soul.
I like YMO's oldschool synthpop a lot. Their first two albums are required listening and massively influential. Definitely check out Susumu Hirasawa and his band from around YMO's time called P-Model.
POLYSICS are pretty poppy, and kind of inconsistent, but they have a ton of catchy, enthusiastic guitar/electro hooks.
Nujabes is a fantastic hip-hop DJ who does some instrumental stuff and works with a lot of Western MCs.
Susumu Yokota, an electronic musician, is a little inconsistent too, but he has some beautiful albums. Check out Symbol and Sakura. Oh, also his collaboration with Rothko, Distant Sounds of Summer.