My favorite arc is the Water 7 arc for sure!
It just has my favorite story. Robin was taken by the World government and the strawhats fight against the World government to get her back. We also learn about Robin's past and that is one of my favorite backstories in one piece. The fights against the CP9 were great and it was the first time where Luffy used his gears.
Also the fight between Usopp and Luffy was great and that Usopp appeared as Sogeking later was a nice part of the story. It even was great when Chopper turned into a Monster
And it was so touching and sad when Merry saved the strawhats one last time before she "died". In the end Robin became a definite member of the Strawhats and Franky was a new nakama. It was also funny when Garp appeared in the end xD
I think I forgot to mention some other cool parts of this arc but I think I explained enough why I like it so much x3