Inspired by Walter Mischel's marshmallow experiment, I'm going to go with delayed gratification. I think the most important skill (or perhaps meta-skill, as this particular skill allows one to develop skills) is the ability to delay gratification and discipline yourself to work on something for a prolonged period of time. Without hard work and discipline, you can't achieve much in life. I also want to link to an interview with Carol Dweck, since she is probably the psychologist who has influenced me the most in this regard.