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.
I was surprised on the amount of Gintama songs in my library versus other anime songs after I finished the anime. That’s when I realized “Yeah, the OST’s that good.”