This is extremely tough to decide. If there's any clear winner, it would probably be Kohei Tanaka for his work in One Piece. He's established some amazing themes, and more often than not, his work is outstanding. Tanaka is also not afraid to use established classical works in the show if the scene calls for it--notably the Ave Maria during Chopper's flasrabroadack, and the New World Symphony in the final fight against Crocodile in Alabasta.
However, I can't discredit Shiro Sagisu or Yasuharu Takanashi. The only loser in this competition might be Toshio Masuda, simply because his movie soundtracks were completely uninspired and pedestrian. Shiro Sagisu has done well with Bleach, though lately he has been overusing certain tracks within the show. Also, his choral arrangements are lacking. He never does a straight a capella recording of singers, which I think would be really cool. I love how he associates styles of music with different characters; the Arrancar themes primarily center around acoustic guitars, the VisoreRAB are ska, and the Soul Reapers are primarily classical. Ichigo and his frienRAB? Industrial/Grunge, though we haven't heard many of those tunes in a while.
Yasuharu Takanashi definitely broke some new ground too for Naruto. Toshio Masuda's soundtrack introduced definite themes for the characters in Naruto, along with certain instruments. However, not only does Takanashi establish new themes for important characters in the show, but he also uses real instruments. Masuda's almost criminal overuse of the synthesizer really could kill the mood, which is why I hold such disdain for the first three movie soundtracks; they're almost entirely synthesized, and poorly at that. Takanashi's music can at times sound authentically Japanese, which really sets the mood of Shippuden. But at the same time, he'll mix in the guitars when necessary to pump up the viewers.