Which was the most important turning point in American history: the ratification of the Articles of Confederation, Shays' Rebellion, or the ratification of the Bill of Rights?
Of the three, I would argue that Shays' Rebellion was the most significant turning point. This rebellion, which broke out in western Massachusetts, demonstrated the weaknesses of the Articles of Confederation to deal with the serious social and political issues that gripped the new United States after the Revolution.
The backcountry Shays rebels, heavily indebted and angry that the eastern political establishment was unresponsive to their issues, rose in rebellion in 1786-1787, closing the courts and threatening to confiscate a stand of arms belonging to the Massachusetts state militia. The rebellion fell apart in the face of a firm response by governor James Boudoin, but powerful Americans realized that the issues causing the rebellion existed in almost every state. Especially horrifying to men like James Madison of Virginia and Robert Morris of Pennsylvania were the debt relief laws passed by many state legislatures that established a looser currency in many states. These were thought to be ruinous to the credit of the states and the nation as a whole, and the government under the Articles of Confederation was essentially powerless to do anything about them. In short, elites perceived an excess of democracy in the nation, and when a call for a convention in Philadelphia to revise the Articles of Confederation went out, many supported the idea. Shays' Rebellion thus contributed to the passage of the new Constitution.
As for the other events, they were certainly important. But the ratification of the Articles established a government that we recognize now was temporary, and the Bill of Rights were ratified more than a year after the Constitution went into effect. In any case, they would not gain their current constitutional significance until a century or so after their ratification.