There is no "most important thing to know". No matter what you know (Russian language skills, how to knit, calculus, the capital of Tibet etc.) the information will be useful at times and totally useless at others.You do need to be able to adapt to changing situations and know when to stand out and when to lay low.
The most important thing any living person knows or pretends not to know " IS THE EXISTENCE OF THE UNKNOWN, OMNIPRESENT"
The most important thing ever is acutally staying healthy and drinking lots of fresh water.