Fond as I am of One Piece, I'd really have to disagree with a lot of what's been said here. People're saying One Piece is a masterpiece because Oda's work is exceptional, because he's a genius, because he's vastly more creative than his compatriots. I would agree that these MAY be true, but really miss the point and what stands out about One Piece. After all, words like "genius" or "masterpiece" are generic terms of praise that any decent fan would tack to their chosen manga. Hell, look at the Manga Review section, where Naruto was just called a unique and compelling masterpiece.
I think what really sets One Piece apart is that Oda, the mangaka, acts like an author. He treats this series like a novel that he's planning out.
Naruto, Bleach, etc...they all have their strong points and it could be argued that they're both masterpieces in their own ways. However, both of their mangaka would absolutely fail out of any writing class/seminar/workshop. They're artists, not authors. And for some, that's just fine. We, on the other hand, clearly appreciate Oda's more novelist approach...I think it adds significant quality and is a whole new technique we rarely see in the manga industry, where artists run the show and are expected to just turn out weekly serials which are supposed to maintain a certain level of dramatic tension in order to retain readers.
See, there are several ways to create dramatic tension, to create interest and intrigue in a story. Most manga lean heavily on shows of awesomness, on pwnsome l33t moments, and crazy action scenes. To my mind, those come off as ridiculously overdone and without anything solid at their core, sort of like a balloon that's been all blown up to a certain size, but doesn't actually have anything inside. One Piece adopts more writing elements, where Oda forgoes the instant-gratification moments and obscene action cliffhangers and carefully builds in a more compelling support structure, plot, and more...novel-y characters. For me, that's kind of like actually BUILDING a paper-mache ball as opposed to blowing a balloon. It takes a lot more time and a lot more effort...and it's going to be MUCH smaller at the beginning. But the mache ball has much more potential for growth and actual substance inside. Each reader has different preferences, of course...my background and perspective have been heavily affected by growing up on a lot of novels/literature; hell, I teach creative writing to middle/high-school students for some quick cash every summer. As a result Naruto, Bleach, etc...just seem kind of empty, vapid, and poorly written based on my tastes and history. I can't really get into the action scenes because I can't get over how...poorly written they are in an AUTHORLY way--though they certainly can amuse in a comic book way. One Piece, on the other hand, is written very...well.
So yes, I do personally think Oda's more creative than his coworkers. But that's not One Piece's greatest asset, in my mind. I think that it's his exceptional style as an author (in that he actually HAS one) that allows him to utilize this creativity; because he approaches this series as an author, rather than just an artist, his creativity can be used to its full potential.
To sum it all up, I think that One Piece is more of a graphic novel series than many of its peers, which gravitate more towards the comic book side of the spectrum. Both are reputable, but I'm a sucker for writing quality, so I love One Piece.
Edited retro-reply to zozo96 so we don't waste posts: Yeah, as a self-styled critic who's blitzed the lit before, I totally get what you mean and I personally agree that Oda...well...is at least extremely talented. Maybe even a genius. However, I think attributing OP's success to his creative talent is a mistake in that it...well...overlooks some of his more crucial abilities and what's more, it enters this realm of subjectivity. After all, genius is a highly overused term that people usually toss out to mean they like something...and what's more, I believe that using that term takes us away from a lot of OP's real values by simply dismissing them as products of Oda's genius. Oh, and it makes the review harder for non-fans to appreciate while deciding whether or not to read. So yes, I agree, but I disagree with the wording and analysis, if that makes any sense?
I believe you could have the most creative person in the world behind a manga and still have it flop miserably if he/she can't write and can't organize his/her thoughts. OP thrives because Oda's skills as a writer allow his creativity to shine through--if he lacked either of the two, One Piece would be a terrible manga. What's more, his work is far more organized than any of his contemporaries'. I don't know if he's ever received formal writing training, but it would make a lot of sense. Plenty of manga writers are creative. Oda's just about the only one I've ever seen that is creative AND can write a story and that's what sets OP apart for me.