The customer

The customer can be internal or external, groups or individuals, and is typically the stakeholder with the most requirements. It's not unusual for project managers in certain industries to have both internal and external customers on a variety of different projects.

It's safe to say that the customer is going to drive the scope of work, drive whether or not the deliverable is accepted, and probably drive you a little crazy as well. That is why it is so important to identify the customer's requirements, keep them informed of your progress in a transparent manner, and identify any impacts on project work if the customer requests any changes.

One of the biggest challenges to project managers is keeping up with the scope requirements when the customer keeps changing what they want. Because the customer will have a high amount of power and interest in the project, it will be crucial to communicate effectively, set expectations, understand expectations, be crystal clear in what success looks like, and manage changes in a formal manner.