Name the most important CONSPIRACY in American history

[Dawn Meredith]

...and name the chief mastermind conspirators or facilitators. I will award 100 points
for the first correct answer.

Jack

This may not be THE most important but the JFK assassination remains the key conspiracy. It is the most studied and the key to understanding what came later, like 9-11.
Everyone will differ on part two of your question. I don't think it can be known and further believe the way it happened is the key, and why it matters.
Dawn

I'd also put Operation Mockingbird at the forefront, given that the sheeple believe we do have a "free press".