I guess we won't know whether the secrets we discover are the dirtiest or not unless the entire range of secrets is somehow dispensed to the world for analysis.
The likelihood of this is so remote that your question can't be answered precisely. The nearest answer is to share some dirty secrets, but not to establish that they are the dirtiest.
The US has hidden many dirty secrets which have eventually come out into the open. And, I’m sure there are more secrets we will learn about in future years. One that I find particularly interesting given President’s Trump’s ongoing war with the “fake media” as he likes to call the American press, is that in 1963, CIA agents spied on several journalists.
The project was called Operation Mockingbird. A CIA report titled “Family Jewels” describing this project was declassified in 2007. This reports talks about wiretapping the homes and offices of two Washington based journalists in order to find out what they were talking about with “senators, members of Congress, and other well-placed individuals”.