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.