It's kind of a toss-up. The anime can be more immersive, but the manga might offer a purer form of the creator's vision. Maybe start with one and see how you like it.
It's a tough call. If you want a more immersive experience with music and animation, go for the anime. But if you prefer a purer form of the story and don't mind black and white art, the manga is a great choice.
It depends on your preferences. If you like to take your time and enjoy detailed art, the manga might be for you. But if you prefer dynamic visuals and voices, the anime could be better.
One Piece currently had a total of 396 episodes. If you want to start watching One Piece again, you can start from episode 396. However, it is recommended to confirm if you have watched all the episodes because each episode of One Piece has a specific storyline and theme. If you haven't finished watching all the episodes, you may need to re-read some episodes that are not involved.
Well, 'The Time - Traveling Straw Hats' is a really cool One Piece fanfiction. In this, the Straw Hat crew somehow gets sent back in time and has to interact with past pirates and events. It's full of twists and turns. Also, 'One Piece: Reimagined Origins' is good as it gives new backstories to some of the main characters, which makes for a very interesting read.
In the world of One Piece, there's no indication that the characters themselves watch fanfiction. Fanfiction is a creation of the fans in the real world, and the in - universe characters are too busy with their own adventures, like Luffy chasing after the One Piece, Zoro training to be the world's greatest swordsman, and Nami navigating the seas. They don't have access to or knowledge of the fan - made stories about them.