Top 10 Best One Piece Story Arcs

Enies Lobby Arc

The arc is Oda's masterpiece. All the amazing setup, plot twists, and epic fights truly make it the greatest arc in all of One Piece.
We had all the stuff with Usopp, Robin, Franky, and, of course, CP9.

This is the arc that made me believe Luffy could become the Pirate King.

Marineford Arc

This is the most brutal and emotional arc.
Luffy goes all out to save his brother Ace from execution. Remember that Luffy couldn't save his crew from separation earlier, so with Ace he is even more determined. Just when Luffy saves him and things seem to turn around, Akainu attacks them. Just before he could kill Luffy, Ace sacrifices himself to save him.
What really gets you is that it's the first time you see Luffy, who is always confident and smiling, completely broken and helpless.
And let's not forget Whitebeard's attack and his inspiring final words confirming the One Piece is real.
This arc is what changed Luffy forever, as he started to realize he and his crew were not strong enough to take on the New World yet.

Alabasta Arc

The first time that every member of the crew gets their own fight, and the first time the big bad is actually a challenge for Luffy.

This is one of my favorites. Every single character had their big emotional moments.
The fight scenes were both epic and emotional.

Water 7 Arc

This arc has the Great Sogeking, from the Land of the Snipers!

Whole Cake Island Arc

In retrospect, it was the best arc post-timeskip (at least as of now), better executed and more emotional than Wano, in my opinion. I know a lot of people have come to the same conclusion too, based on discussion threads about the subject.
It was quality writing and top-notch character development. I wish Oda would do more with the core characters.

Big Mom, Katakuri, the Vinsmokes, tsundere Pudding, Sanji's backstory, Luffy vs. Sanji, epic battles - this arc had it all.
Also, one of the most emotional arcs so far.

Skypia Arc

Best story of all arcs. I don't know how long it takes to think about such a good backstory to an island in the clouds.

This arc does drag in places, but Enel is an incredible villain. He abuses the fear of God to terrify the Skypieans into doing what he wants, with Luffy being perfect to fight him. This allows Enel to be overpowered without needing to come up with a reason for Luffy to still win.

Arlong Park Arc

This one was very interesting. The fight between the Straw Hats and the merman was epic! It was a difficult fight, but in the end, they won, and Coco Village, along with Nami, was set free.

Best arc in One Piece, the reason I'm watching. It is a microcosm of a bigger arc.
The best Nami scene in all of OP happened here. It should get more attention.

Dressrosa Arc

Baratie Arc

Sabaody Archipelago Arc

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Egghead Arc

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Marine Rookie Arc

Thriller Bark Arc

Thriller Bark was unique and amazing. The setting was the best, the characters, especially Perona, the zombies, and how Luffy interacts with them. Brook's backstory was so emotional and sad.
The ending of the arc, where they celebrate and sing Binks' Sake, is amazing and also emotional. Zoro taking Luffy's pain was epic. Absalom wanting to marry Nami was so funny.
The best of all was the setting and Brook's story.