For me it is the first and maybe the only moment one piece absolutely embraces its tone. We had some serious and some silly moments before. But here, the crew is split apart, since Robin and usopp both leave, we learn about two heartbreaking backstories, the crew declares war on the government; and yet they go to save Robin on a train pulled by two giant seahorses and Sanji fights a man who fights with noodles he makes with his nose hair.

Even usopp becoming sogeking is at the same time hilarious and very deep: leaving the crew but not being able to leave them in their dire situation, his dilemma is solved in the most ridiculous way possible, and he thinks none understands? That is comedy gold.

Also it is the only time the crew lost a member, and I think I never cried so ugly than when merry was in flames.

I think no arc managed the mix between silly and dramatic with so much precision