I want to say One Piece. A lot. It's music is very iconic. To the point that I know that when a scene happens in the manga, I can say "Oh I bet they're going to play this music here" and I'm actually usually right. Plus, outside of like one or two, One Piece has a very good track record with OP songs.
But....I have to give it up to Bleach. Especially early Bleach because they gave us the awesomeness that was Number One. One Piece has a great track record with OP songs but Bleach (to me personally) has an even better one mainly because I recognize most of the bands playing. And the battle music for Bleach is very up-tempo when it needs to be and very creepy when it needs to be. I don't anyone can forget the theme music that plays at some points (you know, the one that has the strained guitar strings and the people in the back going "waa-waawaa" all distorted-like). So as much as I think the story is lacking, I'll give it to Bleach in terms of music.
Naruto had great music in the begining but then started to lose it in Shippuden. In terms of OP songs, I think it's ironic that most of the songs I liked were in filler and the canon ones weren't the best I've heard (Except for OP 5 for Sasuke Retrieval arc. That was the greatest) whereas for Shippuden, the canon arc songs rock and the filler arc songs aren't that hot. Especially the current one though I have warmed up to it. FLOW is coming this week so I'm looking forward to that.
Hey, why hasn't FLOW done a One Piece OP yet?
-Kamen Rider Decade-"I'm just a Kamen Rider passing through. Better remember me!"
TOM's final words of Toonami. The proof of what Toonami was aimed for and was meant for. Stay Gold. You will always be first and simply the best.