I would say the turning point was December 23, 1913. At that point, financial control of the United States was transferred from the government, and thus from the people, to a small group of private business owners who have been using it for their own gain ever since.
But even in spite of our flaws, we still strive to be freer than other countries. Even when we fail to live up to that ideal, we still strive for it reaaaaalllly hard. Now, when we can eliminate the institutionalized racism, poverty, sexism, religious bigotry, oppression of gays, warmongering, and mass incarceration of non-violent drug users, then we'll be "the world's most exceptional nation, a beacon of freedom, the good guys and heroes." Though the strength of that position will be diminished if we don't stop lagging behind the rest of the world. We're the slowest in the industrialized world to adopt social equality, like gay marriage. And remember how we took our sweet time abolishing slavery? We really have a lot of work to do.