The phrase "most opening" implies a certain level of greatness or prominence when it comes to the beginning of something. However, there are several antonyms that can be used to describe the opposite of this idea. One could use words like "least significant," "small opening," or "unimpressive start" to convey the opposite meaning. Additionally, words like "closing" or "ending" could be used to describe the opposite of an opening. In the end, the antonyms for "most opening" depend on the context in which the phrase is being used, but they all point in the direction of the opposite of greatness and prominence.