my reasoning for putting those particular characters in the top tier is that they're either:

classic, archetypical yanderes: dai, alice, byakuren, hisami

naturally unhinged and would just get worse : flan, koishi, junko

completely inhuman, eldritch, unknowable: yukari, eirin

fundementally incompatible with the idea of love/incapable of handling it: medi, parsley, seija, kokoro

wildcard: yuuka

Seiga: Seiga