Filler is a necessary evil for long running anime like One Piece, Naruto, and Dragon Ball Super. Whether it expands on the original story to great effect or creates interesting side stories, filler is important to pad out the run time so the anime doesn't catch up with the manga.
Filler has gained a reputation for being boring and just straight up bad. Not all filler falls into this category and there are some truly splendid episodes of One Piece that make the anime worth watching week to week. Some episodes rise to the top and become just as good as the anime canon episodes.
10 Hot Blood Special Unit Participation! Bridge Offensive and Defensive Battle!" - 7.4
After the exciting Skypeia story arc, the straw hats are thrust into the midst of a populated marine base and must work together to survive and escape. This filler arc, lasting 11 episodes is a great example of added material that does not detract from the quality of the overarching narrative. You get a satisfying and entertaining story about the straw hats fighting marines.
These episodes have great action scenes on a bridge as Luffy, Sanji, Zoro, and Usopp battle for their lives against a variety of marine soldiers. This episode is full of great action and funny character interactions as the gang gets ready to fight for their lives against the marines.
9 "The Devil's Fist — A Show Down! Luffy vs. Grount" - 7.5
Grount is a seriously underrated character in One Piece and his rivalry with Luffy is really well done. Grount reveals what's underneath his metal arm and battles Luffy.
This fight is very well animated and is one of the best One Piece filler fights out there. The conclusion of the fight is satisfying and sets up a possible rematch between these two monolithic warriors. What more can a One Piece fan ask for?
8 "A Hungry Front - Luffy and the Marine Rookies!" - 7.5
Luffy and the Straw Hats are starving and they finally spot an island in the distance. To their surprise, the island is a marine base and the Straw Hats must sneak on the island and steal some food for their ship.
This episode introduces All-Front Grount and his rivalry with Luffy. The episode has plenty of humor and explores the different types of people who reside on this base. This episode takes the continuity of the main series (Aokiji) and uses that to establish Grount in the One Piece universe.
7 "Burning Roller Race! Dash into the Final Round!" - 7.5
Seeing as the whole Long Ring Long Island arc already feels like Filler from the main story, it makes sense to add a filler episode to add more content to this arc. Foxy is like Team Rocket in the sense that he mainly played for laughs and his stories are usually more chill than the majority of One Piece. This a good example of a filler adding more content to make the story feel more fleshed out. This episode adds another challenge to the Davy Back fight as the Straw Hats try to roller skate to the death!
There are plenty of creative moments as we see the Straw Hats struggle and overcome this challenge. The way Luffy figures out how to win using his unique abilities is also satisfying. Overall, it’s a really fun episode that shows creativity and fun in the animation and storytelling.
6 "So Long, Marine Base! The Final Battle for Freedom"- 7.5
Jonathan is such an interesting antagonist and is easily one of the best filler villains. Throughout the arc he places traps to trick and outsmart the straw hats, but in this episode it truly shows just how formidable he is by draining the water in the G-8 base and grounding the going Merry.
The use of dials also foreshadows the use they have later in the show, especially when it comes to Usopp's fighting style. The final escape from the G-8 base is a smart and thrilling conclusion to the G-8 arc. This leaves the Straw Hats in a position to head to the next step in their journey as they prepare to meet a certain Foxy pirate.
5 "Luffy and Sanji's Desperate Decision! Large Rescue Maneuvers!" - 7.5
Everybody loves a good prison break! This a great episode that focuses on Sanji and Luffy as they attempt to rescue the rest of their crew. It's great to see Sanji and Luffy banter and work together as we don't get much of that until the Whole Cake arc.
The comedy is on point as poor Special Inspector Shepherd is being labeled as a Straw Hat pirate. Jonathan is further developed as a great antagonist against Luffy as he sets up traps and counterattacks. It's interesting to see Luffy go against a character who is more calculating and level headed.
4 "And Then the Legend Begins! To the End of the Rainbow!" - 7.5
What does it mean to be a truly great pirate? This episode explores what it means to be a pirate as Luffy finishes off his battle with pirate Captain Wetton. Wetton is depicted as a coward who hungers for power and compared to Luffy, who continues to fight with his own powers, proves to us how great of a pirate Luffy will become.
The scene where Zoro tried to cut electricity was hilarious and the ending caps off this story arc very well, even though the time travel is a little contrived.
3 "Angry Finale! Cross the Red Line!" - 7.6
Early One Piece is not as completed as later One Piece. All the challenges and perils are quite quaint when you compare it to the grand line, but the charm of early One Piece is what makes it so fun to watch and that is the strength of this episode. Luffy fights Eric who has the sickle-sickle fruit which makes for a visceral and bloody final bout.
It also explores the millennial dragons and what makes them tick and it adds a lot to the world of One Piece. The episode ends with Apis staying on the island so he can protect the dragons and the Straw Hats start to embark on their journey to enter the Grand Line.
2 "The Implacable Three — A Big Chase After the Straw Hats!" - 7.6
Sometimes all you really need is some big dumb fights and some great animation. The strength of this episode is how it allows us to see a series of interesting fights in such a short time.
All-Hunt Grount is further developed in this episode as we see him fight Luffy and develop his rivalry with Luffy. You also get to see Sanji and the rest of the Straw Hats prepare a rescue plan to get the food. This is a great middle part to this arc that has plenty of action to satisfy the pickiest of anime fans.
1 "A Heartbreaking Duel - Luffy vs. Sanji - Part 1" - 8.2
Flashback episodes are a common way to make filler episodes. The fight between Sanji and Luffy is one of the most iconic episodes of the entire series and the anime made a filler episode to accomplish two tasks. Firstly, to remind the audience of the relationship between these two characters and to prepare us for their emotional fight.
Secondly, for us the see the thoughts and emotions of the characters so we can understand why they will fight and why this matters to them. This episode is a great example of adding to the material the original manga and making the narrative better.