Eiichiro Oda's One Piece started serialization in Shueisha's Weekly Shonen Jump back in 1997 and has gone on to become a mammoth of a series since then. The manga follows what could be called the grandest adventure ever written, in which Monkey D. Luffy sets off on a journey to become the King of the Pirates.
The manga has been going on successfully for 23 years now and is filled with some of the longest arcs, and while the length of the arcs might be intimidating, there's no denying that they're masterfully written.
10 Punk Hazard - 46 Chapters
Punk Hazard arc takes place after the two-year time-skip in One Piece. It starts in chapter 654 and concludes in the 699th chapter of the manga. The Punk Hazard arc sees the Straw Hat Pirates visit their first island in the New World despite not wanting to go there in the first place.
This arc is where the set-up for the big battle vs Kaido begins, and Luffy joins forces with Trafalgar Law to take down Doflamingo, also known as Joker, to ruin Kaido's SMILE business.
9 Thriller Bark - 48 Chapters
The Thriller Bark arc sees the Straw Hat Pirates get caught in the Florian Triangle and lured to a large ship called Thriller Bark. This arc runs from chapter 442 to chapter 489 and thus ranks among the longest One Piece arcs.
Luffy faces off against Gecko Moria in this arc, and the Straw Hat Pirates also have to unite forces to take on the Zombie of the Continent Puller, Oars.
8 Fishman Island - 51 Chapters
Starting in Chapter chapter 603, Fishman Island is an arc built up since the early days of One Piece. Right after the events of Water 7, the Straw Hats are told that their next destination is Fishman Island, however, things happen, and they don't get to go to the island at the time.
After reuniting on Sabaody Archipelago, they finally go to the island and fight off Hordy Jones. Beautifully, they break down the animosity between Humans and Fishman to a great extent before concluding in Chapter 653.
7 Water 7 - 53 Chapters
Water 7 begins in Chapter 322 and ends in the 374th chapter of the manga. This arc is the second one in the Water 7 Saga and is connected to the Enies Lobby arc, which follows its events.
Luffy and the Straw Hat crew arrive at the City of Water in this arc to find a shipwright and fix the Going Merry before moving forward. While doing so, the crew's integrity is tested in the Water 7 arc, and new enemies in the form of CP-9 make their appearance.
6 Enies Lobby - 56 Chapters
Enies Lobby picks up right from where Water 7 left off, in chapter 375 of the manga, and runs up to the 430th chapter. This arc is widely considered one of the greatest in the entire story, and it sees Luffy and his crew go to Enies Lobby, one of the three major World Government strongholds, to rescue Nico Robin.
The arc features some of One Piece's greatest fights and gives the fans a deep look into Nico Robin's past. Without a doubt, Enies Lobby is one of the best-written arcs in the entire story.
5 Alabasta - 63 Chapters
One Piece's Alabasta arc began with Chapter 155 of the manga and continued all the way up to 217, making it one of the longest arcs in the story. It also covered one of the major wars in One Piece as the Kingdom of Alabasta had fallen into turmoil due to a lack of rain.
Crocodile, one of the Seven Warlords of the Sea, was the mastermind behind the entire catastrophe and Luffy and his crew went out of their way to defeat him and restored peace in the country in the process.
4 Skypiea - 66 Chapters
Shortly after the Alabasta arc, One Piece turns towards the Sky Island saga, the main part of which is covered by the Skypiea arc, which runs from Chapter 237-302.
The arc perfectly encapsulates what One Piece is about as Luffy and his crew go on an adventure in the sky, discover the City of Gold, known as Shandora. They also come across God Enel, defeat him and end the centuries-long feud between the Skypieans and Shandians.
3 Whole Cake Island - 78 Chapters
One of the best arcs in One Piece after the two-year time-skip, Whole Cake Island runs from Chapter 825 to 902, and it sees the Straw Hat Pirates go all the way to Totland, the country of one of the Four Emperors of the Sea, Big Mom, to retrieve Sanji.
As the arc progresses, the crew's goal shifts from saving Sanji to helping him rescue his family from the murderous Big Mom. Whole Cake Island also gave fans an incredible fight between Luffy and Katakuri.
2 Wano Country - 92+ Chapters
The Wano Country arc began in chapter 909 of One Piece and is still going on, with the manga reaching its 1000th chapter a few days ago. The arc is divided into Acts, 2 of which have already wrapped up, and the third appears to be in its climax.
It sees the Ninja-Pirate-Mink-Samurai alliance finally go up against Yonko Kaido and Big Mom and liberate Wano Country. This arc is undoubtedly going to become the longest One Piece arc before it ends, but it sits in second place for now.
1 Dressrosa - 102 Chapters
Beginning in Chapter 700 and marking its end in Chapter 801, Dressrosa is the longest arc in One Piece so far. This arc sees the Straw Hat Pirates, Trafalgar Law, and Kin'emon go to Dressrosa to deal with Doflamingo and destroy the Smile factory.
The arc serves as a bigger set up for the Four Emperors Saga and accomplishes quite a lot on its own. After the fall of Doflamingo at the hands of Luffy, the Straw Hat Grand Fleet is also formed.