In case of Scrum, the best choice for Senior User is one, or a few of the Product Owners working in the delivery level, or a representative of them.
Note: based on the Scrum Guide, each project can have only one Product Owner. However, there can be a group of people responsible for product ownership and one of them will be called the Product Owner. In this case, the same person, or other representatives from the committee can be Senior Users.
- Senior User at PRINCE2 wiki