The biggest, most persuasive proof for the Flat Earth model -- indeed, the one bit of evidence that all Round Earthers would insist upon -- is locating The Edge.
The Edge is simply the dropping off end of the flat planet; the corner where the downward sides start, the point where the flat surface ceases. If you are among those who believe the flat planet somehow goes on an infinite plane, then The Edge would be the limit of known geography beyond which everything is terra incognita. Either way it would be something not on the conventional maps of the world, and it ought to be in every direction if you go far enough.
Then, of course, having found The Edge, mankind could explore it. That exploration might encourage new industries, new science, and the discoveries of new resources. Maybe, beyond The Edge, there are plants and animals that human have not yet seen, maybe even creatures comparable to humans with their own culture and sciences.
Let's face it, if the earth were flat then there'd be an Edge, and, if there is no Edge then it follows that the earth is not flat.
Poll