The Marine base filler arc pre-timeskip was mid, but the post-timeskip pre-WCI filler arc was fun. But there are some anime without filler. (JJBA for example). But i think that One Piece anime handles filler well. It's handled in a way that it's not impacting the plot, not retconing anything (normally), and the arcs are spaced out. Plus filler is good, you NEVER want an anime that overcomes the manga.