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.
Are we talking the OPs or like, the score?
For the former, Bungo Stray Dogs and Sword Art Online are easy picks, though they both only used a couple/few artists. Fullmetal Alchemist (both of them) had more variety and a good range. My Hero Academia, too.