If someone wanted to become a corporate controller, what skills would you tell them to master (besides being the best accountant they can be)?

Collaboration, making themselves valuable by guiding instead of blocking the business processes. Get familiar with operations to become a business partner who provides guidance and a framework to operate. Staff is always willing to cooperate more when they understand the "why" behind principles and regulations. 

Influencing and communication skills are critical to work with a broader set of stakeholders

Communication and organizational skills.

Adaptability is important. A controller needs to be flexible and adaptable. This helps them in responding to changes in financial regulations, market conditions, and company needs.

By far the best skill for upper level accounting roles is communication. You will not only need to make the right accounting decisions but explain them to auditors, executives and potentially regulators. The ability to communicate is the difference maker at this level.