Who do you consider to be the most important product stakeholder?

Not every stakeholder has to be invited to a Sprint Review. Which stakeholder would you be certain to invite this Sprint? That's the most important one. Which of them would provide the best feedback for the current Product Increment, and is best positioned to help update the Product Backlog?

Stakeholders are people who are impacted by the product and are interested in the success of the project (could be end user, customer, project manager, CEO etc.). Collaborating with them is very critical for the success of the product. The idea would be list out your stakeholders and do a stake holder mapping exercise (based on interest and influence) and decide how you would best manage them (keep informed, monitor, work closely, keep satisfied). 

When listing out the stakeholders it is very critical to identify the goal of each stakeholder and how aligned are they with the organizational/product goals and how your ongoing product will help them achieve the goal. The key thing is to balance expectations (which might sometimes be conflicting as each have their own priority).

There is no "most important" stakeholder.  There could be a "most important stakeholder right now" as @Ian alludes but none are so important as to be dominate. If you start to rank your stakeholders it is too easy to start listening to some over the others.  In that case you start to build a product that the higher ranked stakeholders want and others may not.  If your organization builds custom solutions then that is ok.  But if you build a mass marketed solution, you will not appeal to the masses if you listen to the "most important" stakeholders.