In general, I think conspiracy theories can be divided between prima facie reasonably plausible and totally implausible. The plausible ones are generally ones that assert a standard 'conspiracy'- shadowy figures in power colluding to do something bad. So for example, I don't personally believe that George Bush did 9/11 or that the CIA killed Kennedy, but if I found out that they had, I wouldn't be shocked to the core, or really have to change my worldview. These things are within the power of the organisations accused, and one can sort of see how they might benefit.
On the other had, there exist conspiracy theories that either require tipping one's worldview on its head (aliens, the Devil, etc.), or would require a conspiracy so huge and all-encompassing that it's just implausible. It just wouldn't be possible to run a huge conspiracy decieving the entire earth, without leaks- governments (people in general!) just aren't that competent. So e.g., flat earth- this would require massive amounts of resources, by a world government, to silence anyone who found anything and make 8 billion people deny the evidence of their own eyes. This would have to employ hunreds of thousands of people- why doesn nobody know one of them? There are seriously no leaks? No 'man in black' ever told his wife on his deathbed? It's implausible. Furthermore, there's no 'quo bono'- in whose interests could it possibly be to spend astronomical resources on convincing people the earth was the wrong shape?
At this point, the die-hard believers start making up whole cosmologies about how the world is an illlusion or it's all aliens or satanists or whatever, at which point it crosses the line from conspiracy theory into schizophrenia.