The ability to make baked ziti is the most important skill a GM can have
I think it is hard to boil down to one thing. Anytime I think of just one skill, I realize that might be important, but it won't float a game on its own. That said, gun to my head, I'd go with the ability to adjust to the table. That might be a bit of a copout answer. But I think GMing is a lot like doing stand-up comedy. You have to refine what you are doing for the table you actually have before you. So knowing how to adjust is an important skill (doesn't mean you have to break style, but you need to know how to bring a broad range of players to what you are doing)