Davy back fight/Long ring Island Arc was the most forgettable for me. Despite being a canon arc in the manga, it still end up feeling like filler to me. That's how I know its my least favorite arc.
Also, almost every arc posttimeskip have dragged on for too long. Fishman island, Dressrossa, WCI and now Wano. These arcs all have amazing moments, but the poor pacing can seriously drag down the quality of an arc. Sometimes Oda has a lot of exposition and lore, plot points that are used to great affect in the story telling and world building, other times they just feel like they end up going nowhere and a complete waste of time.
There were arcs pre time skip that had the same problem (Alabasta and Skypiea for example), but those were still shorter than the arcs are now.