I disagree, partially at least. You're right about the Avengers theme, I think, and plenty of the others have been good but not super-memorable or just not quite well-developed enough. But I find the music of Dr. Strange, Guardians of the Galaxy, Winter Solider, Civil War, Thor: Ragnarok and Ant-man just as good and memorable as the music of Black Panther and I don't think any of them are really far off at all from Williams' Superman or Elfman's Batman. I don't think those Williams/Elfman themes are at all on some rareified level of quality that is almost never reached. They're classic and catchy and fun, but the biggest reason they stayed king of the superhero theme music world for decades is because almost nobody was doing any even particularly good work in the superhero theme music world for decades.
Personally, I think the Ant-man theme is actually every bit as musically memorable and potentially iconic as those older themes, with the only major problem with it being that Ant-man just isn't as iconic as a character and his movies just aren't as broadly liked. And I would also add Werewolf by Night, WandaVision, Loki and Shang-Chi as also having genuinely beautiful music in the MCU.