Effective communication. Communication is used in almost every educated profession, and even outside of careers, being able to effectively communicate with other humans is crucial to developing a lot of other skills and just generally being successful at life.
However, perhaps even more important is self-discipline. I don’t know if this necessarily a “skill,” but it is an ever-important trait that will guarantee you success. I’ve spent a LOT of time pondering over the meaning of life, and ultimately I concluded that the masters of life aren’t the rich billionaires in mansions; they’re the homeless dudes wanting 50 cents to buy a drink, and then happily talk to you for 45 minutes. They’re the vagabonds that train hop everywhere because they feel like it. They’re the poor folks on the edge of town in a run-down tiny apartment that have to shoplift to feed themselves. THOSE are the real masters of life. And, as I discovered, the key to that can be summed up in “don’t have what you want.” Therefore, self-discipline may also be the most important skill.
Sorry if that was too much a rant. I’ve just spent, like, a LONG time (try hours a day, every day, for YEARS) thinking about that.