Copywriting. The fine art of being able to move people solely with your words. If you're good at copywriting, you can basically write your own ticket to your future. Good copywriters have the ability to get people to do what they want them to, when they want them to do it. This is a huge asset.
This is the best business skill you can learn. Great copywriters aren't born, but learn and evolve. The best can command top rates easily and usually write for corporations or ghostwrite for the best marketers out there. Those that hire them will invest thousands or tens of thousands of dollars to have them write a single sales pages or multiple pages because it's a very good investment and will generate thousands or even millions of dollars in revenue.
I've been asked numerous times. What's the best skill I can learn for running my own business and make my bottom line better? This is always my answer. If you're a good copywriter and can emotionally move people with your words and create trust, you're bound to be successful in whatever you choose.