It has to be Water 7/Enies Lobby.
There's just so many things I love about this arc
- Firstly that it focuses heavily on best Strawhat Robin who puts her entry for most TRAGIC PAST into the heavily competitive One Piece backstory competition.
- Ussop gets some nice development, there's an element of foolishness to his actions but you can get why he feels the way he does for the ship having seen what the others haven't. The ensuing Sogeking silliness (always gets me how Luffy and Chopper are just so dense to not see through the obvious disguise) and the man's admittance of having fucked up at the arcs very end makes for a good piece of character progression.
- Arguable second best Strawhat introduced via Franky
- CP9, and more specifically Rob Lucci, makes a good ol' fashioned power game foil to Luffy, is incredibly gratifying to watch him get gatling'd through a wall.
- Spandam is such a wonderful cowardly arse heel villain, having this mostly all talk type of character be technically ranked above the actual challenging foes of CP9 is a nice twist, I mean sure it's nothing that hasn't been done before but in One Piece this kind of character is usually small time fodder, he is also rocking the Mankind Mick Foley mask.
- There's a freaking Elephant Sword (or Sword elephant?) called FUNKFREED
- "I wanna live!" enough said, potential GOAT One Piece scene right there.
- Kaku justifying the awesome powers of the giraffe, even if it's initially just to convince himself he didn't get a shitty power.
- they'll make you cry over a fuckin' ship
- As an anime bonus it gets two of the best opening themes
Runner up to Sabaody Archipelago for all the reasons outlined by Imperial Bishop