Which event was the most important for orderly growth of the nation?

It is difficult to answer this question, because there have been a number of events that were important. For one, the creation of the Constitution, including the Bill of Rights, meant that this country would be governed by the rule of law and would be a democracy rather than a monarchy. Then, there was the acquisition of various pieces of land -- for example, the Northwest Ordinance provided a framework for the creation of between three and five new states, and the Louisiana Purchase doubled the size of the United States and contributed to the expansion of the country.