Antonyms for the word "most heart-warming" include phrases such as "most heartless," "most discouraging," "most uninviting," "most intolerable," and "most disheartening." These phrases contrast with the optimism and positivity associated with "most heart-warming" and paint a picture of negativity and despair. They can be used to describe situations, people, or actions that are unkind, unfeeling, and insensitive. When looking for an antonym to "most heart-warming," it's important to consider the context and how it will affect the overall tone of the sentence or paragraph.