Flat Earth is not and was never science. Scientists/philosphers have known the earth is a sphere (or oblate spheroid) since at least the ancient Greeks.
The best video is the one with the flat earthers renting a boat, a powerful laser and actually set up a really nice experiment to prove that curvature doesn't exist. They were smart enough to find a setup that was scientifically accurate, but sadly it showed that the Earth indeed has curvature. The poor sods then thought they did something wrong :(
Well they did. They started with the results they wanted to achieve and worked backwards.
Having watched that documentary, I can only assume the entire flat Earth "movement" was an attempt by that lonely guy to get into the milfy redhead's pants...