The antonyms for the word "most nonformal" are formal, strict, rigid, conventional, and traditional. These words indicate a structured, organized, and regulated approach, in contrast to the relaxed, casual, and easy-going style of nonformal. A formal setting often involves specific dress codes, etiquette, and procedures, such as in a corporate office or a government institution. Strict and rigid imply a strict adherence to rules and regulations, with no room for deviation or flexibility. Conventional and traditional indicate a preference for established practices and customs, rather than innovation or experimentation. Thus, the antonyms of "most nonformal" reflect a more rigid and traditional approach to communication, behavior, and interactions.