I want to watch One Piece, but the anime has a lot of filler content not in the manga. I'd rather not watch the filler content and only watch the episodes which are directly related to the plot. To be clear, I'm defining a filler episode as one which is not based on any story in the manga or based on extra manga chapters which have nothing to do with the overarching story.
Which episodes are filler?
This question is taken from this question. I was curious about this myself after completing the One Piece manga, but to keep uniformity between similar questions, I took the same wording as Logan-M had used in his question about Bleach filler.