Far from it. If you want to go by sales, One Piece rules, by popularity in japan, One Piece rules. By storyline and content, that's a different story. If we're just narrowing it down to shounen action manga, then you have quite a few contenders. One Piece is ofen viewed as the King of WSJ, then you have Bleach though it's gotten very shit so generally it isn't viewed as a contender, then you have up and coming series like Beelzebub and Toriko, Toriko seems to be pushed by WSJ to take over Bleach's top three spot. Then you have sleeping giants like Hunter x Hunter, a manga that could easily rival One Piece and possibly succeed it should it return from hiatus with stable weekly releases and an ongoing anime, something that I think may happen soon as special chapters of HxH have been released recently and it got a new anime several months back. Then you have finished series like Full metal Alchemist Brotherhood, Yu Yu Hakusho Ruronui Kenshin and Dragon Ball. As for anime you have Code Geass, which easily trumps the majority of animes out there, including Naruto imo.