The antonyms of the phrase "most well built" could be "weakly constructed," "shabby," "ramshackle," "dilapidated," "flimsy," and "fragile." These words convey the opposite meaning of strength, resilience, durability, and sturdiness that "most well built" implies. A building or structure that is weakly constructed may not last long and may be prone to damage, collapse, or wear and tear. Any structure that is shabby or ramshackle may not be safe to reside or work in, making it unsuitable for habitation. The presence of dilapidation, flimsiness, or fragility in a building suggests a need for repair or renovation to prevent further damage and ensure safety.