5. Reverse Mountain/Laboon/Whiskey Peak: I consider all of those parts one story arc. What I liked is that the Strawhats got into the Grand Line. It also introduced the funniest One Piece pair: Mr. 9 & Vivi, and one of my favorite One Piece villain groups, the Baroque Works. I have to say the biggest highlight of this arc was Zoro vs. the Baroque Works millions.
4. Arlong Park: I've always liked the internal struggles the Straw Hats went through. We really didn't know much about Nami, so I like how Oda revealed her association with Arlong, and why she was such a petty theif. Her flashback was one of the saddests moments in the series. And one of the best parts: Usopp standing up for himself when he beat Chuu.
3. Little Garden: To be honest with you, I thought this arc would be one of my least favorite ones. But as soon as Dory and Broggy inspired Usopp to be the Warrior of the Sea he's been aiming for, the whole story changed. Mr. 3 became one of my favorite Baroque Works agents after seeing what an intelligent and cunning villain he was. It was cool to see how Usopp ended up saving the crew and proved his skills. I also liked how Sanji talked to Crocodile and ended up stealing the Eternal Pose even though he wasn't shown that much.
2. Alabasta: One of the most touching arcs in the series. Even though the crossing the desert part dragged on, we still found out about Vivi's past, and more about Baroque Works. I started to like the arc more when the crew arrived at Rainbase. We really see how much of a bastard Crocodile really is, and how he kept saying that nothing could stop the war. Sanji's Mr. Prince counterpart was entertaining, too.
1. Water 7/Enies Lobby: Another internal Struggle arc. The crew is hit by one disaster after another as soon as they arrive: Robin dissapearing, finding out that the Going Merry is beyond repair, getting their money stolen, and Usopp leaving the crew. I hated Franky for what he did to Usopp at first, but after he kidnapped Usopp, I liked their bonding. The CP9 revelation was one of the most surprising scenes. The Puffing Tom part had its share of moments: Sogeking, Franky showing off his moves, and Sanji vs. Wanze. From the moment Luffy stormed into Enies Lobby asking where Robin was, I knew this arc would be epic. Even though the Strawhats vs. the CP9 wasn't as good as the Alabasta fights, they had their moments, especially Luffy revealing the Gears, and Chopper's "Monster" point.