Best: Water 7. It's the peak of One Piece story-wise IMO. The strawhats face their first major crisis with the departure of 2 crewmembers, then we have THE greatest villain reveal of the series (CP9). The follow-up (Enies Lobby) also had great story moments and probably the most intense battles of OP.
Worst: Wano. An incredible mess from start to end. Almost everything was disappointing: the main villain (just another mindless bully), the flashback (Oden's idiotic story), the side characters, the Nika retcon. It's an irrefutable proof that Oda simply can't write about large-scale conflicts.