Oh, there are so many conspiracy theories out there, from small to large. It's difficult to choose one.
If I had to to choose one particular aspect of some conspiracy theories that I find particularly amusing and interesting, then perhaps it would be those that are really, really hard to distinguish from parodies (in other words, where
Poe's Law is in full effect.)
A prime example of this would be flat earthers. When you read their arguments, it really reads like a parody, and a quite poor parody at that. It's not even all that funny if it
were a parody. However, apparently at least
some people take those arguments seriously and believe them.
It's quite amusing to see to what lengths they go to try to dismiss simple everyday experience that any person can experience themselves, and the level of global conspiracy that would be required to keep up the things they claim.
Inventing their own fictitious laws of physics is a common staple among most (if not even all) conspiracy theories, but not many go to the absolutely ridiculous lengths as flat earthers. Basically
nothing we know about physics is true, according to them.