At the moment I have to say Nakano followed by Tamada, Haga and Kiyo. I think Kiyo will make his way up to MotoGP aboard a Honda. They like to have at least one Japanese rider filling out their bikes and I fear Tamada's time may be running out. Unfortunately I don't think Haga will make a return to GPs, given he never had an ultra competitive bike, although the 2001 Red Bull Yamaha was no dog by any means, he never did a whole lot. I think he will finish out his career in WSB and hopefully grabs himself a title.
Just for the record the greatest Japanese rider of all time was Daijiro Kato.
Oh, to answer your question Ron... No Japanese rider has ever won the 500cc/MotoGP World Çhampionship.