One Piece - Enies Lobby
Wow. I don't even know if this can be considered a story arc on its own. It's basically a huge climax to Water 7. Starts off feeling like a literal One Piece Musou comic, tons of action, not a lot of plot, huge stakes, then it interleaves all the various characters and plot elements developed in Water 7 into layer after layer of conclusions, with subplots and backstories being tied up and expanded. Really phenomenal storytelling. I loved Nico Robin's flashback, and the legacy of the Lost History is becoming more and more intriguing. The climax to this arc was a total tearjerker. It's really amazing how sad One Piece can be for a manga where no one in the actual story ever "dies". Hats off to Oda for weaving a story where a ship "dying" can be that powerful. Amazing.
The epilogue after Enies Lobby was fantastic too. Seeing all those characters again and watching the reactions from all the places the the Straw Hats have been to before, seeing the bounty grow, getting the new ship, and getting all sorts of crazy foreshadowing on The New World, the Ds, and all that Ace vs Blackbeard stuff... omg...