The word "crueltiest" refers to someone or something that is excessively harsh or unkind. Synonyms for this word might include "most vicious," "most barbaric," "most sadistic," or "most ruthless." These words all describe a level of brutality or callousness that is beyond what is considered normal or acceptable. Other synonyms might include "most monstrous," "most inhumane," or "most brutal." Each of these words captures a different nuance of the concept of cruelty, but they all communicate the idea that the subject is particularly severe or extreme in its unkindness or brutality.