My least favourite conspiracy is Flat Earth because it's just so absurdly stupid. My favourite though is one that doesn't get a lot of attention - the one that says 1000 years have been added to our history. I first read about it through a russian chess grand master who was a proponent. I haven't been able to find his original article but I have found this video about it : https://youtu.be/IKy3xPsgky0