Here are my preferences:
1)
Skypiea:
Certainly, Skypiea was slow in the Noland's flashback and all the protagonists facing the Death Game. But the feeling for so many catholicism countries to defy God in a completely new landscape depicting Heaven to end with the protagonist with the power of the devil holding himself the title of the King of the Seas before the showdown. Epic in the most raw way to take it.
Skypiea is, indeed, my favorite arc, it holds many ingredients to be added in any outcome possible for a place in the series. The religion involved also delivers a great taste, usually it's needed courage to write about it, despite any resemblance to start comparing and pissing people.
2) Alabasta:
Just like above, but regarding politics. The first kingdom to appear and try to be taken, it had flaws but was quick phased and the feelings were flowing non-stop. Zoro's fight was iconic and Crocodile is one of the villain most easy to despise yet being awesome for his haughtness.
3)Totland:
This is specially because of the indirect revelation of what the One Piece is, which it makes you part of your reading. It's fast, holds plenty of information and clues to use in the future, it's a big wall and it's a woman leading the hierarchy. Makes me wonder if Luffy will be hitting a woman for a third time.
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Wano arc will be the best
But, since that probably isn't allowed, I'll vote...
I think you still can because Wa no Kuni, indeed, has appeared. Just check how the trees in this picture are made:
They're just like japanese type of drawing would depict them. You can talk about how Apoo is communicating information, reactions like in the pic and how X Drake is working... also how it's defined already as a winter island, etc.