ok, I don't know what brought up this notion that black Glover has the best-written side characters but that's bull.
well... it's safe to assume that the characters in black clover are clearly written with a sufficient amount of personality and development in them but it's totally wrong to say they are the best written in anime.
like... just think of how wide anime is, to have watched so little of them yet claim one to be the very best in anime is just ignorant.
animes with side characters with development on par or greater than black clover are;
joker game
one piece
Moriarty the Patriot
Rainbow
and so many much more, so much that it would be a waste of time to list them all.
just to make it clear the notion of black clover having the best-written side character is false