What happens as you investigate conspiracy theories is you start to question the mainstream media more and more, then you start to question the "alt-media", then you start to question history, science, evolution, religion, existence. When you get to that point you go back and and watch the mainstream media again and you start to see how it's all bullshit.

"We'll know our disinformation program is complete when everything the American public believes is false."

William Casey, CIA Director, 1981

Once you realize that the media is bullshitting to you constantly, if you then want to understand how a certain thing actually happened, you need to examine the evidence from the null hypothesis.Take the JFK assassination for example; if we understand that the media lied to us about the assassination and it actually was a conspiracy, then if you take it to the next level you might begin to wonder whether the information they gave you on him dying was actually even correct.

http://mileswmathis.com/barindex2.pdf

https://welovecarbondioxide.wordpress.com/2015/03/30/the-hidden-king-camelot-ruled-from-the-cave-of-merlin-by-miles-mathis/

Now that one's a hard pill to swallow for most of us but I used it as an example as to where questioning everything can lead you.

As for your question, the most plausible ones are the ones that are actually confirmed but no one talks about and totally ignored by the majority of the people who dismiss conspiracy theorists as crackpots.

https://www.reddit.com/r/conspiracy/wiki/locc

One of the most important confirmed conspiracies that one should be aware of is Operation Paperclip. Over 1500 Nazi scientists were bought over under the guise of "rocket scientists", but instead were the most adept propagandists from the party many of which went on to join the CIA.

The CIA was formed through the efforts of Dulles and Prescott Bush (who profited from selling oil to both sides of the war). This act alone alludes to the fundamental intention of the CIA as a supranational organization, that doesn't necessarily have the best interests of the American population at heart.

When you start to research some the propaganda techniques employed by the Nazi's and the contributions of Freud to this field, you see the role Edward Bernays (Freud's nephew) played in the establishment of the Public Relations industry (propaganda for propaganda), where Neuro Linguistic Programming is at today, and what the CIA was getting up to under MK ULTRA, you might start to be able to entertain the idea that you have been lied to about MUCH MUCH more than you believe.

That is the most plausible conspiracy theory really. That you have been lied to to a much greater extent than you can possibly imagine. The rest of the dominoes fall once you start to appreciate that.