Final answer:
The best example of Montesquieu's ideas in a US founding document is the United States Constitution, which reflects his principles of separation of powers and checks and balances.
Step-by-step explanation:
The best example of Montesquieu's ideas in a US founding document is the United States Constitution. Montesquieu argued for the separation of powers, which is reflected in the Constitution's assignment of executive, legislative, and judicial functions to different branches of government. This system of checks and balances ensures that no single branch can overreach its power and maintains political liberty.