Okay, let me tell you One piece is simply the best Anime/manga ever because of many things. The one thing is that it's fun. Like realy, it's hilarious xd Even in very sad situations they can pull something fun, completely random. It has the best characters that you recognize yourself in, and it's very emotional. It's sad, I won't lie and say it isn't, it's the saddest anime ever. Try and take the saddest moment of Naruto, and make the pain 5 times bigger. That's One Piece. It's sad, but fun xd. A thing that makes it completely different from many Animes, is that the main character is not the strongest. This makes it very interesting, cause some times some baddaz people appears and just tear everything apart. And when the main character isn't the strongest, you get a very exciting excperience since you don't know what will happen.
The mysterious characters. Dragon and Shanks.... Two of the most baddas characters in anime like ever, the only thing is... They almost never appear. Dragon has appeared 2 times in the anime I think, but still everyone loves him. 10 minnutes of screening is enough to make the wiewers know that he is epic and to be feared. Then you have those who always runs around but are shit xd Buggy the clown for excample. One of the first enemies. You think: "okay, Buggy lost, that's the end of him. New enemy!", but that's not how it is. Oda (the writer of OP), never, and I repeat, NEVER forget about a character. You may think that everything is over, your fav character is about to die but BAM! Comes a character you haven't seen in 200 episodes... It's so epic. The main character isn't always your fav, and let's not forget the villains xd The villains in OP are so epic that sometimes you cath yourself cheering for them! Also, the enemy today might be the ally tomorow

It's so strange, in OP the villain Luffy defeat in one episode, might be his ally in another. Thus you never know what will happen.
The storyline and moral: Well, in Naruto you have the friendship is most important thing and all. That's how it is. The only moral in Naruto is that the kids where alone but they worked it out, and grew to be loved. In OP, you have that aswell, only alot more intense. We do not get to know only about Luffy's childhood, but all the other characters past. Some has been alone from the start, some saw their home got burned away in front of them, some got thrown away of their family and the weapon they used killed the only man who cared for him, one lost his friend, one was driven away by all, his family and all, untill someone took him in... And so on.
Sry for the long post xd If you realy wanna watch OP, I hope you do

It's so freakin epic, and I could've writen you a book about it xd