I am considering both the openings/endings and the ost. The usage and impact of the music can elevate any piece of media in general. For me the best music has to be in either Naruto or Bleach, ops and eds were bangers, the osts were iconic and so freakin good. The music really elevated the mid adaptations we got. AOT is close third, it might be my bias but its freakin aot, everything is perfect. Honorable mention to Your name, tokyo ghoul, black clover, your lie in april and other music animes, fmab, HxH, Noragami, Frieren, monogatari, etc.