Those are all more Sagas than arcs. An arc is a smaller part of a saga, like Jaya arc inside the Skypiea Saga, or the Baratie arc inside the East Blue saga. But, the "CP9 Saga", or as I prefer to say, the World Government Saga was my fav, just for Post-Enies Lobby arc and Enies Lobby arc.
I didn't really like Skypiea very much. I liked it at first, but then it started getting on my nerves with all that tension and everybody fighting everybody and the God-Enel-is-a-bastard-that-wants-to-destroy-this-island-but-everybody-is-too-afraid-to-even-speak-of-him thing. And Luffy vs Enel was cool, but I've seen better fights. I just liked it 'cuz Luffy really put his brains to work in that fight and didn't win just 'cause he's rubber.
CP9 Saga is my favourite, technically because of the Enies lobby arc due to it having some of its fights being the type of fighting that I am used to seeing when it comes to battles in many of the anime/manga series that I had seen in my lifetime, not to mention that I am a very big fan of fictional martial arts.
you really got to know and get used to vivi
east blue is 2nd because that was the making of the straw hats we know today and plus an original concept before the whole who's got the strongest df power and such
My favourite arc is now Thriller Bark arc. There is so many action with funny scenes.. i just love it =) and i hope Brook will join too. But i like the Enies Lobby arc too, because everybody showed some art of a new technique.
I liked the Enies Lobby arc since it was a crucial "step-stone" of the SH crew.. It powered them up a lot, made their bonds stronger and even showed to what extent they will to go even only for one nakama. Not to mention the post-enies-lobby arc was also nice.. =D
I like the Enies Lobby arc as well. Luffy and the crew made improvements in their strength + Franky as the new member of the group. To be honest, I hated the Skypiea arc. It was incredibly and ridiculously boring. I watched the arc through the anime but the manga version itself wasn't much better. Throughout Skypiea, I felt as if I was watching a bunch of fillers. After more than half way through the arc I decided to skip and move onto the next saga. I picked off where the crew first met Aokiji and the serie got interesting again.
All the arcs were awesome, but i found skypiea less satisfying then the others mainly because no one of the pirates, shichibukai, Yonkou or the World Government knew about this trip so no bounties were raised, no gained ''respect'' and ''fear'' or whatsoever, but the arc had some awesome moments. Greatest disapointing moment was that Enel didn't got beaten, you got this major arc which ran for 42 episodes and 65 chapters and the guy just flew away.. CP9 was in my opinion one of the (more) better mainly because it took the time to develop such a great plot, introducing Water 7 to conspiracy theory, to CP9 and World Governement, just freaking awesome, credits to Oda.