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.
Has no one watched Beck Mongolian Chop Squad? The anime is about a commercially successful rock band. The producers had to license a solid soundtrack of songs for scenes of the characters performing. The soundtrack is one banger after another.
Yet, somehow FLCL still beats it.