The antonyms for the word "most serendipitous" would be "less fortunate", "unlucky", "unfortunate", "unplanned", "disadvantageous", or "unpredictable". These words are the exact opposite of serendipitous, which means "fortunate happenstance" or "pleasant surprise". When something is described as serendipitous, it implies that it was unexpected and brought about something positive. However, the antonyms listed above indicate a lack of luck, planning, or positivity. It's important to remember that life is a mixture of serendipitous and not-so-serendipitous moments, and it's how one reacts to these moments that can make a difference.