My top 3:
1. Chemtrails
2. Climate Change Conspiracy
3. Moon Landing Conspiracy
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For me, the stupidest one is the Big Bang Theory. On paper, sure you can believe it. But when you look outside, up at the moon, glowing in your face, the stars and lights on the planes flying overhead, do people REALLY think that anything like the Big Bang could have really happened? There are limits to science, and sometimes, it's best to not try and find out how EVERYTHING came to be. Just to leave a sense of wonder.