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.
I once did some research on flat earthers for a project. I was trying to assemble the most consistent interpretation I could in order to play the devil’s advocate in a fun debate. It turns out that space denial is quite helpful at preserving coherent physics if you assume the Earth is flat. The Earth can be accelerating upwards to create gravity, which violates special relativity but that’s nbd at this scale. Other planets can also be flat, which makes things easy too. The sun can’t set, but there’s no way around that one.
In my experience, the most irrefutably evidence that the Earth is round is the simple stuff like astronomy. Simple things like the path the sun and stars take from two points on the globe basically ends all debate. Flat Earthers always have crackpot ideas for things you can’t easily observe yourself, but you can really catch them with proof they can see with their naked eyes.
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.