When looking for antonyms for the phrase "most mild-tempered," we can consider words that mean the opposite of gentle or calm. Some words that fit this description include aggressive, confrontational, irritable, and volatile. These words describe a personality that is prone to outbursts, conflict, and impatience, rather than one that is composed and peaceable. Other possible antonyms include harsh, severe, and cruel, which describe a person who lacks kindness and empathy towards others. By considering these opposing words, we can better understand the qualities that make someone mild-tempered and appreciate their value in building positive relationships and creating a harmonious environment.