There are a lot of things one should learn for a content life. As parents, we are tempted to ignore in our teachings many things that we've already mastered and not struggling with, but which are very important nevertheless. I'd be cautious to tag something specific as "the most valuable thing" only for this reason. There's always a balance, trade-offs all in between.

For your example, you can teach your children how to be very successful at making money and achieve an accomplished financial situation, then they still may get abused somehow through some weakness you left them out with in your parental education.