... you have a backlog of PBIs and need ordering criteria.
Scrum says to prioritize the items in the product backlog. The backlog must be a single, ordered list of PBIs (Product Backlog Items) to bring focus to the development team, and the entire enterprise, on the deliverables. The goal of the Product Backlog is to optimize ROI (return on investment) in the long term.
You could order these by the priorities of your dominant customer. However, that might lock out other customers, and in any case tends to focus on revenues rather than costs. Since cost is also a key consideration in ROI, focusing on customers alone isn't enough.
You could order these by the priorities of the Product Owner in isolation. But the question arises: What criteria should the Produc Owner use to set the priorities of the Product Backlog Items?
You could order these according to the team's insight on the development dependencies between items, but that misses out on major revenue-generating opportunities that may arise for PBIs that may not be tightly constrained.
Therefore: The Product Owner orders PBIs in a way that generates the largest long-term ROI. The product owner is empowered to make decisions on behalf of all the stakeholders, but would be wise to balance the supplications and insights of all stakeholders in balancing the ROI equation.
This evaluation rarely can reduce to a formula. Such ordering is always imperfect and benefits from good judgment, insight, and experience.
The notion of priority for PBIs is flawed. The term priority is an attribute of an individual element of a collection that is to be ordered. However, we are focused on ROI rather than a token quantity, such as a priority, in ordering the Product Backlog. There often can be no linear correspondence between priority and ROI. You can't sit back and assign context-free priorities to PBIs and then sort the Product Backlog, because a PBI's ROI is often a function of its position in the backlog. Some market offerings make sense only if they build on previous offerings; some market offerings are sensitive to market timing. Re-ordering items can change the ROI contribution of each one; in fact, Jeff Sutherland notes that you can as much as double your long-term ROI just by re-ordering the PBIs.
ROI needn't be just about money. In general, it is about value. Most value systems are rooted in the economic theory of value: the ability to have exchange of one artifact for another of equal value. However, an enterprise may value employee retention, good customer relations, a good public image, or something else. Any item of value can be viewed as a return on some kind of investment. See Team Sprint.
To serve process improvement it is best that ROI be measurable.