Final answer:
The Patriot Act, passed in response to the 9/11 attacks, is considered the most important counterterrorism law in the United States.
Explanation:
The most important counterterrorism law ever passed by Congress is the Patriot Act. This law, officially known as the Uniting and Strengthening America by Providing Appropriate Tools Required to Intercept and Obstruct Terrorism Act of 2001, significantly expanded federal powers to monitor electronic communications in the aftermath of the 9/11 attacks.
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