With One Piece's recent debut of chapter 1051 released Sunday, June 5th, 2022, a major event in the series just occurred, and while many fans held speculation that it was just a matter of time, its reveal will nonetheless be massively important to the remainder of One Piece.
As One Piece continues its serialization, many of its previous arcs tend to be forgotten years after their initial telling. Though all of One Piece is exceptional, certain arcs are significantly more important based upon their impact on the story and the major events within them.
Sabaody Archipelago Arc
The Sabaody Archipelago Arc acts as the introduction of the Summit War Arc where the major powers of the World Government convene to discuss and debate the happenings of the world. The Straw Hats travel to Sabaody by mistake when they are attempting to reach Fish-man Island without knowing of its strategic placement.
With Sabaody placed before the entrance to The Grand Line, it serves as a pirate gathering spot, and while the Straw Hats are on Sabaody, they meet other members of the Supernovas or "Worst Generation." They also personally draw the hatred of Jewelry "Big Eater" Bonney after causing mass havoc by hitting a World Noble, which could lead to Bonney attempting to kill the Straw Hats further down the Grand Line.
Thriller Bark
Thriller Bark was originally an island that was later converted into One Piece's largest pirate ship of the same name. It also serves as the extension to the spooky theme of The Florian Triangle where the Straw Hats first meet and recruit the undead pirate musician Brook.
While Thriller Bark's arc is relatively short, it is packed with foreshadowing for later events in the series. The country of Wano is foreshadowed through the Zombie Ryoma and the sword Shusui which he bequeaths to Zoro. Thriller Bark is also used to set up the reveal of Kaido and Big Mom, members of the 4 emperors and major future enemies of the Straw Hats.
Arlong Park
Arlong Park is the stronghold of the Fishman Pirate Captain Arlong, who chose the location near Cocoyasi village with the intention of bending them to his rule. The Straw Hats travel to Cocoyasi Island after Nami steals the Going Merry, and the crew is determined to get both Nami and the ship back.
Arlong Park is arguably the best Straw Hat backstory revelation, with Nami truly joining the crew at its conclusion. Though she recently betrayed them, Luffy immediately forgives her as he understands her struggle with her past. It also contains one of Luffy's most powerful displays of friendship when he leaves Nami in the care of his infamous hat in one of One Piece's most touching scenes.
Dressrosa
The Dressrosa Arc set on the island of Dressrosa ruled by Donquixote Doflamingo, one of One Piece's strongest warlords, continues the events of the Punk Hazard Arc. After learning that the horrible experiments conducted by Caesar Clown on Punk Hazard were done at the request of Doflamingo, the Straw Hats travel to Dressrosa to beat Doflamingo and stop the production and shipment of SMILE.
The greatest result of these events would be the creation of the Straw Hat Grand Fleet. While the main Straw Hat crew alone is a formidable force, the Grand Fleet foreshadows the need for much bigger crews in the battles to come when the Straw Hats will have to fight against the rival fleets of the 4 Emperors and Marines.
Post-War-Arc
After the events of Marineford which saw the loss of many pirates including Ace, Whitebeard and Luffy nearly dying, the Post-War-Arc serves to fill in the events of the following 2 year time skip. The Post-War-Arc also ends the first half of the series, leading to the dynamics between Marines and Pirates changing and the 12 Supernovas and 4 Emperors openly competing with one another.
The choice to reveal Luffy's backstory with his brothers Ace and Sabo served to both establish the past as well as foreshadow the future revelation of Sabo and his importance to the story. It also served as a brief respite for readers who were still recovering from the events of Marineford while also continuing the story gradually.
Water 7 & Enies Lobby
Following the Straw Hats' confrontation with Enel in Skypeia, the crew realizes The Going Merry is in dire need of repairs and chooses to dock at the closest island they can find, Water 7. Upon reaching Water 7, the world government becomes far more prominent and starts to truly hunt down the Straw Hats for their connection to Nico Robin.
The Straw Hats face some of their most trying tribulations during Water 7 and Enies Lobby. While the crew is torn between respecting Robin's wish of leaving her behind and escaping over their wish to save her, a rare moment where each Straw Hat lays their feelings bare was an excellent use of character development. The arc's climax of Robin crying her eyes out and finally begging to be saved by her friends was her most emotional arc to date.
Alabasta
The desert Kingdom of Alabasta, ruled by King Nefertari Cobra, is revealed to be manipulated by the crime syndicate Baroque Works, who used propaganda to turn the kingdom's people against the royal family. By teaming up with Nefertari Vivi, a Princess of Alabasta, the Straw Hats aim to restore the Kingdom to its rightful ruler Nefertari Cobra.
Alabasta is still debated solely on its introduction of Nefertari Vivi. The conclusion of Alabasta, in which Luffy named Vivi an honorary Straw Hat, has made Vivi one of the most popular characters in all of One Piece. Vivi's introduction and decision to remain behind also foreshadowed her importance among the members of the world government, and the ensuing war that it will potentially lead to in the final arcs of One Piece.
Whole Cake Island
Whole Cake Island is the first Island the Straw Hats encounter after leaving the Sabaody Archipelago and entering the New World. Whole Cake Island is ruled by the Emperor Big Mom and serves as the base for her family and her Grand Fleet, captained by her many sons and daughters.
The events of Whole Cake Island rank among some of the greatest fights in the series. It showcased the true power of an Emperor on their home turf and the wild fights showed how far Luffy still has to go. With the knowledge that the Emperors will be some of the strongest final enemies in the series, Big Mom's fighting prowess lends much excitement to upcoming and unbelievably large war arcs.
Marine Ford
With Blackbeard having captured Ace from Impel Down and surrendered him to the Marines, the Straw Hats and Whitebeard Grand Fleet pit all of their allies against the entire consolidated forces of the Marines and their most powerful members. With everything on the line, the Straw Hats fight to their utmost capabilities and show have far they have progressed from the beginning of the series.
With 25 chapters, most of which are double-page spreads of highly detailed fight scenes, Marineford is easily the biggest war arc in One Piece and potentially the biggest war arc of manga in general. Every page conveys the utter desperation of Luffy and Co.'s attempts to recover Ace, and each blow only adds to the suspense of finally reaching Ace.
Wano Country Arc
After barely managing to escape from Whole Cake Island and Big Mom, the Straw Hats make their way to Wano in their attempt to bring Kinemon and Momonosuke home. However, they soon learn that Wano is currently ruled by yet another Pirate Emperor, Kaido of the Beast Pirates, one of One Piece's most powerful villains. The Straw Hats then agree to form an alliance in their attempts to dethrone Kaido and restore Wano to its former glory.
The Wano arc is the culmination of everything that has made One Piece great so far. With fights between not one but two emperors, new allies and crew members, and a payoff for hundreds of chapters of foreshadowing, Wano has become the most densely packed arc in all of One Piece and has been one of the most well-written arcs to date.