One Piece: Baron Omatsuri and the Secret Island (2005)

Solidly marries the comedy, camaraderie, and joyful spontaneity of the series' earlier, less worldbuilt arcs with the eldritch tension of cult-commune and monster horror, from a time when the darkness in One Piece was less historic and more personal. Narratively it's nothing special, but why would you watch a One Piece film for the thesis? Visuals: sinister and spectacular. Music: relatively deft and restrained. Animation: really impactful and well-cut. The literal squash and stretch of Luffy's body in the final-act fights is so powerful!