As one of the most popular and long-running shonen manga and anime, One Piece has also been adapted into many different films. While none of these movies have been canon to the manga, they have have each helped give more life and sometimes even context to the story as a whole.
This relationship to the source material -- specifically how much it expands on it -- is one of the most crucial aspects to the overall quality of the films. It's what sets the tenth One Piece movie, One Piece: Strong World, apart from the nine that came before it -- especially since the series' creator Eiichiro Oda had a hand in crafting its story and villain.
The Plot of One Piece: Strong World
One Piece: Strong World debuted in 2009, and is the last One Piece film set before the timeskip. Taking place between Thriller Bark and the Sabaody Archipelago arcs, it follows the Straw Hats encountering a forgotten legend in the pirate world named Shiki the Golden Lion. While this character was made for the film, Oda wished he had incorporated Shiki into the manga's story so badly that the anime incorporated his origins into the Impel Down arc. This helped create further hype for both the character and the then-upcoming film.
The plot revolves around Shiki, one of Gol D. Roger's greatest rivals, kidnapping Nami for her navigational skills and taking her to his floating islands in the sky, which he plans on dropping onto the East Blue as a grand show of force. Naturally, the Straw Hats go after Shiki to save their crew member but the villain splits them up into separate groups, making the rescue attempt of both Nami and the majority of crew's home islands much harder.
While the damsel in distress motif isn't necessarily unheard of, especially in One Piece, Strong World really goes out of its way to showcase just how powerful Nami is as a character. What she lacks in brawn she always makes up for with brains, and this is made incredibly apparent throughout this story -- she is vitally important in ultimately foiling Shiki's plan.
Where to Watch One Piece: Strong World
One Piece: Strong World perfectly balances the roles of the other Straw Hats, with the pairings both making sense and giving them appropriate amounts of screentime. The Sanji/Usopp pairing in particular is very entertaining to watch, as well as Zoro's nonexistent sense of direction putting Chopper in hilarious situations. Luffy's dynamic with Nami is highly evocative of Arlong Park, which for many fans was the storyline that made them fall in love with manga/anime.
Shiki's ability to telekinetically lift and move non-living objects of any size is both dangerous and a great spectacle for the eyes. His power sets the foundation for the film in that it is responsible for the entire plot. The Straw Hats wouldn't be able to fight on floating islands, nor would Shiki's threat be as great without these abilities. It's all very evocative of the Skypeia arc, especially with how Luffy has to face another godlike figure on a floating island.
Strong World might not be as easy to jump into as other movies made from the property, but it very clearly celebrates everything about pre-timeskip One Piece. It can be streamed right now through the Microsoft Store in both sub and dub, while the DVD/Blu-Ray can be found on most online marketplaces.
Other Great One Piece Films
Just because Strong World is the best doesn't mean that there aren't other One Piece movies that come close to its greatness. One Piece: Film Z is considered by plenty of fans to be the best film in the franchise and was the first one released after Strong World. It follows the Straw Hats' journey to stop a former Marine Admiral from destroying the New World after he attacks their crew.
Fans enjoyed this film largely due to how the villain, Z, is portrayed. His backstory is sympathetic (rather rare with One Piece villains) and his motivations are clearly defined. His origin as a Marine Admiral who trained most of the officers fans were familiar with -- as well as being friends with series favorites like Garp and Sengoku -- make Z feel like an actual part of the story rather than a non-canon movie villain. His relationship with Aokiji is especially touching.
Another One Piece film that could almost take the crown is Baron Omatsuri and the Secret Island. What makes this one so great is how different it is from those that came before and after. While there is an actual villain in the film, the main antagonist is really the deterioration of the Straw Hats' relationship with one another. The anime and manga almost always portray the crew's bond as unbreakable, but Baron Omatsuri really puts them to the test. It's not just Sanji and Zoro fighting but everyone, giving the film a legitimately darker tone.
The animation style for this film is also quite different from what the anime had done before. While it wasn't for everyone, many fans did enjoy the change and it helps make the film stand out from all the others in the One Piece franchise.