The opposite of "most nasty-tempered" would be "most amiable" or "most pleasant." Someone who is amiable is inclined to be friendly and agreeable, and they typically have a pleasant and genial disposition. When we interact with them, we often feel at ease and enjoy their company. An individual who is unpleasant or nasty tempered, on the other hand, has a sour, irritable demeanor. They are often rude or unfriendly, making it difficult for others to approach or connect with them. In short, the opposite of nasty tempered are those who are delightful, congenial, and easygoing.