Success in life is determined not only by the skills one has, but by which skills are deemed important and constantly practiced and refined, and which are pegged as unnecessary and eventually forgotten.
Two of the most important skills a person can have are the ability to communicate effectively and the capacity to get along with others, and despite any disagreements or any enmity in general, be able to work with people efficiently.
Both of these skills are not something to be learned from a book in a single afternoon on your front porch. They require many experiences working with people, and the aforementioned skills will slowly build up, if one attempts to improve at every opportunity.
An aptitude for effective communication is one of the hardest things to learn, but very important. Even important world leaders (e.g. Presidents, Prime Ministers, UN officials) often lead countries into conflict, simply because they were unable to effectively convey the meaning of their words! A candidate for student council will never be elected if they cannot tell their fellow students how they plan to go about improving the school, no matter how grandiose their plans might be. Effective communication is useful in areas besides politics as well. On a ship for example, communication is paramount to a smooth ride, and possibly even survival, in the case of a storm.
There are many skills that are important to learn and are helpful in many situations, but focusing on these two will lead to the most success in the widest variety of areas.