The antonyms for the word "most soft-hearted" are numerous, and they all describe a lack of emotional sensitivity or compassion. Some possible antonyms include callous, unfeeling, cold-hearted, hard-hearted, indifferent, unsympathetic, and uncompassionate. These words describe people who are unresponsive to the needs and feelings of others, and who lack empathy and kindness. They may be cruel or dismissive of those who are suffering or in pain. It is essential to use these antonyms with caution, as they can have negative connotations and suggest a lack of humanity. In contrast, being soft-hearted is a positive trait that emphasizes kindness, empathy, and tolerance.