1. Piano: Mitsuko Uchida is a clear first choice. Her box of the Mozart sonatas remains the best. Oddly I don’t like her much in the rest of the classical repertoire, though her Debussy and Webern and Schoenberg are interesting (though not my preferred versions for the latter two, which are the steelier Pollini and Gould). I also like Aki Takahashi, most of all for Cage and Feldman.
2. Conductor: Seiji Ozawa has remarkable talent and he can conduct almost anything without a score (not easy). Still, he never really developed his own sound and he has to count as a missed opportunity. First prize goes to Maasaki Suzuki, who has recorded a remarkable all-Japanese St. Matthew’s Passion and is doing a cycle of the Bach cantatas.
3. String Quartet: Tokyo is first-rate, get their complete box of Beethoven’s String Quartets.
4. Composer: Toru Takemitsu is the obvious choice, though I don’t much come back to his work.
Outside of classical music I’ll recommend Kodo (and indeed all Taiko music, but only live, not on disc), The Brilliant Green’s "The Angel Song," and yes Yoko Ono. Most of Japanese popular music is a blur to me, though not an unpleasant one.