What is the most valuable skill a person can have for their entire life?
The ability to express your thoughts.
Seriously, I believe this is something most of us lack. It's extremely important for a person to learn to put into words, what he thinks. It makes a relationship last. It creates an impression on the person you're talking to. It gives you a chance to explore what others think about your ideas. It lets you answer questions on Quora in a way other people understand.
Although I'm no expert, I can say that you can always develop this skill simply by learning to talk without fear. Talk to a stranger, a fellow passenger on a train or your child-hood crush, they won't mind (I'm not sure about your crush, though). That way, you'll be forced to express your thoughts as clearly as possible. And that's exactly how you learn the art of expressing yourself and your ideas!
So, go out, talk to someone. Unleash your mind. Express yourself!
The ability to tell a good story - If most people are honest, the people who they remember most in their lives are people who could tell them a great story. You are going to meet a lot of people in an average lifetime and while you'll forget most of them, you'll always remember the good storytellers.
The skill is to motivate oneself constantly and to never lose track of one's goals and to work constantly towards them. Mastering oneself is the key to achieving one's dreams and the only way one can master oneself is through self-discipline.
The ability to accept.
Accept that life can't always be the way you want.
Accept that everyone in the world can't behave the way you want them to.
Accept that you can't keep everyone happy.
Accept that unrealistic goals can never be achieved.
Accept that you can't change things which are not in your hands.
Accept that worrying won't do any good.
Accept that your happiness lies in your hands.
If we accept these, life will be much simpler.