Welcome to all the new people. I take back what I said about PC having a small One Piece fanbase. :P
I see three new topics since I last ventured here, so I'll answer them all. First, my favourite arc is probably Arabasta, as I thought Crocodile's schemes were genius, in an evil kind of way. The thought of two sides fighting each other benevolently with the identical goal in mind...that'll always get to me. I also liked that moment with Vivi standing above the fighting, while trying desperately and unsuccessfully to put an end to the violence.
As for my favourite fight, after some time sitting here and thinking about it, I'll go with Luffy VS Crocodile. Yes, I love the Arabasta arc, shoot me. Crocodile was evil, and a man I loved to hate, so watching Luffy pwn him was awesome. And now that he's a free man again, I would be lying if I said I didn't think he'd get a second chance to be pwned. :P
And my favourite Supernova? I'd normally go with Zoro, but seeing as Neiko Star took him and Luffy out of the running, I'll hold off on my judgement. I haven't seen enough of each of them to make an opinion yet, though from what I have seen, probably Law or Bonney.