The antonyms for "most tenderhearted" are callous, heartless, insensitive, unfeeling, and cold-blooded. These words describe people who lack empathy and are indifferent to others' emotions, pain, and suffering. They are unable to sympathize with others, and their actions are often cruel and unkind. Unlike tenderhearted individuals who are compassionate, caring, and nurturing, callous, heartless, insensitive, unfeeling, and cold-blooded people can be hostile, selfish, and unapproachable. In a world where kindness and empathy are essential, it is crucial to cultivate these qualities and develop a tender heart towards others.