What is the most important [defensive/related] training you have received? A particular course, part of a course, a specific subject matter spanning several courses, etc. What is that training which you value most, and why?

For me, it was situational awareness (SA). Learning about SA laid the foundation for pretty much every other aspect of defensive training I've taken since. Standard military training laid the foundation, and then through several specialty courses--mil, LE, and private sector--honed it into something that is useable and quite beneficial in everyday life.

...so, what is that one course, subject, etc. that you would recommend above and before all else for someone else to take?