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For me it's easier to pick the less hot parts of the show, those would be the Skypiea, Thriller Bark and Davy Back Fight arcs. They weren't terrible but by the standards of Arlong, Alabasta, Water 7, Enies Lobby, Impel Down and the Whitebeard Wars they just weren't as good.
Still, the lowest parts of One Piece are still miles better than the high parts of it's nearest rivals.