Skypiea/Jaya easily
Every single great thing about One Piece is captured in this arc perfectly. It's got a great villain, beautiful scenery, interesting lore, introduces Blackbeard in an incredibly memorable way, and just has the peak of the spirit of adventure and creativity One Piece is known for. The only thing I dislike about it is the "lol everyone who looked like they died is fine" ending, but even that is typical Oda.