The antonyms for the word "most prearranged" are "least unplanned." While "most prearranged" suggests a high degree of organization and planning, "least unplanned" implies the opposite. This could refer to something that is highly spontaneous, impulsive, or improvised. For example, a party that is "most prearranged" might have a strict schedule of events and a detailed guest list, while a party that is "least unplanned" might involve a last-minute decision to gather with friends and no specific agenda. Ultimately, the antonyms for "most prearranged" can help to highlight the importance of balance between structure and flexibility in our lives.