For me, it’s Gintama. It’s iconic, easily identifiable, and makes sense on a musical level. On the last point, I’m not knowledgeable on the theory, but the Gintama soundtrack, in general, is enjoyable to listen too. If I have to pick one favorite song there, it’s Katsura’s theme.
Samurai Champloo by a country mile.
Runners up are Carole and Tuesday, Vivy: Fluorite Eyes Song, and Land of the Lustrous