OnePiece

Skypiea is a fun one for sure. and I regard Along Park pretty highly as its what made me definitely fall in love with the anime.

Pre-Time Skip: Water 7/Enies Lobby - entirely for the full package of great story (crazy lows and highs!), worldbuilding, and character development.

Post: Whole Cake and Wano (*Wano would win for me if pacing at Onigashima didn't detract so heavily.)

I'm all about having as many possible avenues of goodness as possible, so good in-arc characters, worldbuilding and lore, and story intrigue come first for me.

Next is uniqueness of presentation style. This is high in both WCI and Wano in particular. It's the least important to me, but all else being equal (or better), the higher animation budgets and special soundtrack additions for both arcs definitely didn't hurt.

Egghead has its own cute style (and colour palette) too but it's mostly just like the manga and only the animation stands out for me, so not much to say there yet. Even for someone like me who hasn't cared to pay close attention to animation quality in the past, I'm starting to feel spoiled by the attention to detail at a lot of key points, though.

(I would have liked to include Dressrosa as there's much to love there, but its downsides are too strong for me.)

*edit: I thought you were only asking anime, lol. For manga, my fave is Egghead for now. Wild situation and insane stakes!

Anime: Water 7

Made in a time before the SFX and OST got old, and right before Toei stopped caring about pacing.

This would probably be a later arc but Toei hasn't kept up their game. The animation goes hard, recently, but it's utterly ruined by the re-use of 20-year-old audio samples and tracks.

Manga: Probably Fish-Man Island

They are a lot more consistent in the manga for me, as each arc isn't troubled by whatever production woes Toei is having at the time.