Which amendment was the most important to the Anti-Federalists?

The Anti-Federalists were a coalition of powerful politicians, farmers, and ordinary Americans that opposed the idea of a strong central government and therefore opposed the ratification of the Constitution. One main point that the Federalists and Anti-Federalists clashed on was the existence of a Bill of Rights. The Federalists argued that there was no need for a Bill of Rights, as any powers not given to the federal government would automatically be ceded to states and individuals. The Anti-Federalists believed it was necessary to protect states' rights. 
James Madison, a famous politician and a member of the House of Representatives at the time, presented a list of amendments that would put limits on the powers of the federal government. What the Anti-Federalists feared most was a tyrannical central government growing too powerful and dissolving the states. The fact that they were spurning British tyranny in the colonies validated their fear. Out of the amendments listed in the Bill of Rights, the tenth was the most important to the Anti-Federalists. It explicitly states that that any powers not given to the federal government in the Constitution go to the states or the people. 
https://billofrightsinstitute.org/founding-documents/bill-of-rights/

https://www.ushistory.org/us/16b.asp