It is difficult, if not impossible, to identify a single biggest achievement from Franklin's long and impressive life. However, a possible candidate for the title of "most important" is Franklin's Autobiography. His scientific discoveries fundamentally affected the way we live, but other scientists in Franklin's era were making similar, if less momentous, discoveries. It seems plausible that someone else would have made Franklin's discoveries had he not made them. No one else in America or Europe, however, expressed Franklin's unique vision of self-improvement and self-reinvention. These values have become cornerstones of modern life, and of American life in particular. They have motivated people to head West for a new life among deserts, mountains and prairies. They have motivated immigrants from poor countries all over the world to seek their fortune in America and other wealthy countries. They influence the way our political leaders talk and act. None of this is directly the result of Franklin's book, but it is the result of a uniquely American mindset that was and is best expressed in Franklin's book. (Keep in mind that there are clearly other possible answers for this question.)