Ok, you're talking with someone that follows One Piece for... 9 years now... So, this is clearly a bias statement. One Piece is just great, it has a really slow start for most people (until the characters reach the grand line, i believe chapter 100 of the manga and episode 75 of the anime).
But is the one manga that with each new saga it gets even better, or at least stays in a very good spot quality wise, It also has a lot of care towards consistency, and good writing. Though i have to say, the anime is not the best way to enjoy the series.
Kept hearing about it.