The word "most prankish" refers to someone who is mischievous and playful, perhaps even a bit troublesome. However, in contrast to this, some antonyms for the word could be "serious," "responsible," or "sober." These words indicate a person who is more thoughtful and cautious in their actions, and less likely to engage in impulsive pranks. Other antonyms could include "respectful," "polite," and "well-behaved," which suggest a person who is considerate of others and adheres to social norms. Ultimately, the choice of antonyms will depend on the context in which the word "most prankish" is being used and the desired tone or message of the sentence.