The term 'most substantive' conveys the idea of being the most important, meaningful, or significant. Thus, antonyms for this phrase would imply concepts that are less important, insignificant, or trivial. Some antonyms for 'most substantive' include trivial, insignificant, unimportant, irrelevant, immaterial, and inconsequential. These concepts indicate a lack of substance or weight, and they are typically used to describe minor or irrelevant details. Therefore, when we encounter antonyms for 'most substantive,' we should consider them within the context of what is essential or fundamental, and what is not.