The phrase "most fallen-in" implies something that is deeply collapsed or deteriorated. The antonyms of this phrase would be words that connote strength, stability, or construction. Words like well-built, sturdy, strong, and stable all represent antonyms for "most fallen-in". You could also use terms such as thriving, flourishing, successful, and booming, as they all connote the opposite of a structure or building that has fallen in. When looking for antonyms for "most fallen-in", it is helpful to think of words that are the opposite of something that has deteriorated or fallen apart.