The most important skills in planning are decision-making as it is necessary to set the right objectives and to choose further direction and, of course, conceptual skills as they are used to choose the right conception how to promote or produce particular good or service.
The most important skills in organizing are time management as it helps to allocate efforts efficiently and to save time, and technical skills needed to set the chain of actions for plan implementing.
The most important skill in leading is, certainly, human relations as when leading you are often faced with cross-cultural problems and you need effective communication skills to resolve possible conflicting situations.
The most important skill in controlling is technical skill needed to evaluate the performed work and to find errors.
Frame, Davidson. (2003). Managing Projects in Organizations: How to Make the Best Use of Time, Techniques, and People. San Francisco: Jossey-Bass.