Which conspiracy theory is so believable that it just might be true?

There are many that are very convincing. I think it comes down to what you want to believe. With enough false facts and circumstantial evidence a person can make almost any claim sound truthful.

I don't find any of them to be credible.

The first thing I always think about is how it could ever remain secret considering the amount of people needed to pull it off. Is it reasonable to assume all those people have kept their mouths shut for decades?

Turns out other people wondered about that as well. The physicist David Robert Grimes estimated the time it would take for the conspiracy to be exposed based on the number of people involved. He based his calculations on known conspiracies (including the Tuskegee Syphyiis Experiment and the NSA surveillance program) all of which were exposed by leaks from people involved in the conspiracies.

To fake the moon landing would have required over 400,000 people and probably would have been exposed in about three years. Climate change fraud would need the same amount of people and been exposed in about the same time. A vaccination conspiracy would need about 22,000 people (not counting the drug companies) and would be exposed between 3 and 30 years depending upon how many patients were involved.

It seems that conspiracies are like affairs, people live to talk about them,

If I were Bush, I would have pushed the button on 9/11 if I wasn't complicit.

Global warming