1- Water Seven/Enies Lobby: Simply Peak Piece! The crew arguing between each other in a serious way for the first and only time,real consequences like the destruction of the Going Merry,the wonderful "I want to live" and declaration of war to the World Government scene. Definitely Peak Piece!
2-Jaya/Skypea: An arc that represent the "dreamer" spirit of One Piece. An original idea of a island in the sky,and a relatively realistic way to arrive there (not for magic or something,but taking the right wave etc.),a completely different population,with a different currency,and great action scenes with good coreography
3-Ace Rescue arc (from Sabaody to post-Marineford): An arc that sees Luffy as the only protagonist without his crew,but still a great arc. Real consequences (From Luffy having to be separate from his crew for a long time,to Ace's death),all the most powerful characters of the manga at the time present,and a lot of plot twists. I used to complain about Marineford,but its actually not so bad as a war. It aged well imho
4-Baroque Works Arc (from Lavoon to Alabasta): The first "save the princess,save the kingdom" arc. And the best one at doing this formula! Vivi is the best princess between the many princesses of the story,and the Alabasta population has a good reason to enter in a civil war,something pivotal like water! And Crocodile is a great villain
5-East Blue Arc: Arlong is a great villain,and the scene where Luffy puts the strawhat on Nami is surely iconic,but apart from Arlong Park which is a great arc,the other arcs of this group are not so good
6-Thriller Bark: I mean..its funny,and the Tim Burton style is cool,but Moria is not a great villain. The ending part with Kuma and Zoro though is surely great
7-Davy Back Fight: I mean,yeah,its funny,but come on! It deserves the last place anyway. Love Foxy,Polluce and the others,though