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.
Some good suggestions here already. I’ll add FLCL to the mix. The Pillows’ youthful, energetic, and sometimes wistful punk rock fits perfectly with the show’s themes of childish rebellion and the pains of growing up.
Oh yeah!