This is a very subjective question. Some could argue that it was the signing of the Declaration of Independence in 1776. Some argue that it was the ratification of the Constitution. Others argue that it was the victories at Fort McHenry, and New Orleans in the War of 1812. Still others argue it was the Civil War, or the Spanish American War. Some even suggest it was key victories in World War II, or winning the Cold War in 1988, or even Barack Obama's winning the 2008 Presidential Election.
My personal opinion is that the Ratification of the Constitution, June 21, 1788, was the most important event. The Constitution transformed the loose coalition of colonies that had recently won independence from Great Britain into the cohesive nation that is known today as the United States of America. Before the Constitution, the country was fractured and weak under the Articles of Confederation. Protectionist economic policies kept the states apart, and the weakness of the over-arching national government made it more part of the problem than a solution. The Constitution created a new national government with the strength to resolve problems between the states, but limited its power such that it didn't interfere in the internal affairs of the states. The result was a strong freedom loving nation that became the envy of the world.
However, this is just my opinion. The Declaration of Independence, the War of 1812, the Civil War and the end of the Cold War are also arguably the most important events, and definitely fill out the top five most important events in US History.