How are they defining "greatest" exactly? No conspiracy theory is great, except in the depth of its lunacy, so maybe that's it. Or maybe they're going by how well-known a theory is by the general public. By either reckoning, they picked the proper #1: the nutty and unsubstantiated claims of the 9/11 Truth Movement.
The classics all made the list: the Moon landing hoax, the JFK assassination, the looming North American Union; plus one or two I'd never heard of. Who knew, for instance, that the U.S. government has half a million plastic coffins piled up outside Atlanta for when they start executing the population en masse? I mean, I'd heard all about the plans to herd us all into secret concentration camps, but plastic coffins? Where are our tax dollars going, exactly?
Here are a few of my personal favorites that didn't make the list: Lee Harvey Oswald shot John Kennedy all by himself, the 9/11 terrorist attacks were carried out by members of Al-Qaeda, and we actually landed on the Moon half a dozen times from 1969 to 1972.