One Piece is pretty much the successor to Dragon Ball aka the number one manga in recent years.
If you’re curious about the reason, first ask yourself; have you watched the anime?
That said, there are so many factors as to why One Piece is considered the best long-running anime.
1. Charismatic Protagonist
Luffy is cheerful, friendly and happy-go-lucky.
He seems simple most of the time, but he’s got incredible luck and intuition.
He fights for friends and befriends his enemies. He will never give up even when the odds are completely stacked against him.
You’ll want to be his friend; even better, you want to be him.
The list of applications to his pirate crew must be pretty damn long.
2. Charismatic Supporting Characters
All the characters in One Piece have their own distinct personalities, traits and quirks.
Just in Luffy’s crew alone, you have a badass swordsman, a lustful cook, a cowardly ne’er-do-well, a cuddly deer-monster-man, a money-minded navigator, a cool and mysterious historian, a funky topless cyborg mechanic and a skeleton for a musician.
How badass is that? That’s only his crew; they meet tons more wonderful and diverse people who are all interesting and completely funny.
3. Wonderful, Fantastic World
The world of One Piece, like its characters, is diverse, unpredictable, and a joy to visit.
It’s like Alice in Wonderland, except that it’s oceans of pirates and marines.
4. They Level Up The Hard Way
The Strawhat Pirates didn’t turn into aces overnight.
They have to resolve on determination, hard work, teamwork and ingenuity to overcome otherwise impossible enemies and scenarios.
Often they get beaten within an inch of their lives. Mostly they already are.
You can’t help but root for them, unless you’re cold-blooded. Or just jaded.
Or if they are fighting a really cool enemy, you might want to switch sides for a while.
5. Romance Dawn
The entire adventure of One Piece is about the Romance Dawn – the heroic quest that sends the protagonist on an epic journey.
The One Piece that Luffy (and everyone else) searches for is a real MacGuffin.
This story is really about the journey, not the destination, and it’s a really epic journey at that.
In short, One Piece contains all the essential elements of shounen manga storyteling.
You follow the characters as they grow through hardship and happiness, joy and loss. It’s often heartwarming and hilarious; sometimes it’s heartbreaking.
It’s a good story with good characters.
(source: Lee Kwok Onn@quora)