When it comes to antonyms for the phrase "most heavy-duty," there are a few options. One opposite word for heavy-duty could be lightweight, which refers to something much lighter in weight and construction. Another possible antonym for heavy-duty is fragile, which implies something that is easily breakable and delicate. you could also use the phrase "least heavy-duty" or "less heavy-duty" as antonyms to "most heavy-duty." Additionally, "weak" or "insubstantial" could also complement this term, indicating something without strength or weight-bearing capacity. Ultimately, determining the best antonym for "most heavy-duty" depends on the specific context and usage of the term.