One Piece... is just one decades long shonen battling series among others. Who likes what best and others not that much is a matter of opinion but saying that One Piece somehow objectively shines above all others is just masqueraded fanboyism.

Story? It is stupid IMHO. The main hero Luffy just one day decides without any real reason that he's gonna become the KING OF PIRATES and be the person to find the mythical treasure One Piece that countless others have tried in the past and present. Yes he has a special power, but so do many others and I'm sure he couldn't be fully unaware about that. So how can he be so confident in himself? Making the character 'stupid' is not an excuse for poor character development IMHO. Which in turn is no excuse for a simplistic plot.

After the average beginning the series goes totally downhill. Nowadays, it is too focused in inventing new sidequests after another, new islands after another, new strange powers after another so that the main story progresses nowhere in even a hundred chapters. Also every story arc is more and more comedy and less substance, which i suppose is the main reason why One Piece is slightly more popular than Naruto or Bleach, which may take themselves too seriously. But I for one like seriousness... So...

Most characters rely only on their personality disorders with doing things, and their role in the series becomes clear immediately when they're introduced. And it barely ever changes. Nothing surprising ever happens character-development-wise. Even the cowardly Usopp's henshin into somewhat more heroic SogeKing totally fits his tendency to put up blatant lie shows so it's not real development. Side-characters likewise consist only of their 1) design 2) powers 3) a quirk or few.

The battles are IMHO duller than those in Naruto or Bleach. This is partly because the decisive move is usually something unbelievably weird stunt that only a person living in a quixotic world after watching too much comedy shows in a marathon would think of pulling out. Another reason is the concept of Devil Fruits which I liked and found having much potential. This potential however is wasted because some of the characters also have superhuman strength the origin of which is never explained, and their strength builds up linearly as the series progresses, and hence a character without Devil Fruit powers can be as strong as one having them, and a character with the powers can be much stronger or weaker physically than another user. This is stupid, and very generic.

Also some people claim it's objectively better that there's no knowing beforehand how two of the powers will do against each other. I think it's just a matter of taste, and like said above, even the potential for this to be really interesting is wasted, because the Fruit powers play only a little role in the grand scale. If a character loses Devil Fruit -wise they still can win by physical strength, and almost always do so. There simply is never the same kind of 'gaming' experience or excitement you find in the tournament arcs of some other shonen series, and good grief One Piece doesn't have one, it would be too boring for me to continue.

The only reason I gave it overall score that is "bad" instead of "average" is because there weren't any before, unlike with Naruto or Bleach which have many. It's not unreadable if you like series like it, but originality-wise it's definitely 1 like Naruto and Bleach.