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.
Given that I just let a shipping company extort me just so that I could have the complete ARIA vinyl collection I’m gonna have to say ARIA for me 😅.
Gintama is also a great choice! It such a diverse anime and the OST reflects that. I love listening to it as well! Picking a favorite is hard but I think it is 昔の友人が変わらずにいるというのも悪くないものだな… It’s very OG and has such an wholesome bittersweet sensation to it. Gintama has a lot of bittersweet and I love that. Life is bittersweet after all. My favorite character theme is probably Madao’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.”