This would be much better as a discussion question because there are so many possible answers. The most obvious answers would be the Revolutionary War or the Civil War. I, however, will go against the grain and argue that the most important event in US history was the election of 1800.
The reason for this is that this election represented the first handing over of power from political party to another. It had been important when Washington stepped down and gave up power, but at least he was handing it to his own vice president. In 1800, by contrast, the Federalists had to hand power over to a man (Thomas Jefferson) that they accused of being an atheist who would completely ruin the country, introducing all sorts of horrible and immoral practices straight out of the French Revolution.
In many new countries, this sort of situation (with Jefferson winning the election) would have led to the government calling out the army to overrule the election results. This could have led to the sorts of protracted civil wars between liberals and conservatives that Mexico, for example, underwent. Instead, Adams and the Federalists stepped down peacefully. That shaped the US into the country that it is today, where democratic elections peacefully transfer power and political violence is essentially unknown.