The biggest conspiracy in the history of the world in my opinion was the founding of the Fed, and it's not even a secret. It's right out in the open, albeit enshrouded in obfuscation and apologetics. But basically everyone agrees that a small group of the richest men in the world, powerful bankers, got together and wrote the Federal Reserve Act, granting them a banking cartel that has shaped the course of world history ever since. Pretty interesting stuff.
Rothbard gave a great lecture on the founding of the Fed:
http://mises.org:88/Rothbard-Fed