Interesting in this context is completely divorced from morally good/bad. Could be any group from any area at any time in history. I’ll start with a few, followers of the cult of pythagoras, contemporary black Hebrew Israelites, antiracist skinheads and the Amish (neo-luddites in general). Don’t be racist or a prick to other people discussing.
Of course, the easiest simplest killshot on flat earth is the sun setting below the horizon. Flat earth and space deniers tend to say the sun is well, something that’s not a star, but more importantly that it stays in the sky and circles further away from a given location on earth until we can’t see it from a given location anymore while it stays overhead. To think that is to deny easily observed reality of it setting below the horizon. They just wordsalad about perspective until the conversation ends.
One aspect of flat earth/space denial that keeps me engaged is learning from the responses that educated people give to their questions. Details of space programs and astronomy that wouldn’t come up often except to answer flat earth/space denial.