Knowledge is gained through insights. While a full understanding and implications of an insight can take a lifetime, an insight can be triggered in under an hour.
Now which insights are most valuable? That is always going to be personal and relative. But one of the insights that touches all people and is almost universally lacking or deeply misunderstood is the insight into money. What is it exactly, where does it come from, who controls it? ...
So that would be my choice. Spend an hour learning about money. Here is a decent and easy starter in under an hour.
https://www.youtube.com/watch?v=bE8i-4HpKlM&list=PLyl80QTKi0...