The customer is often wrong, or abusive, or clueless. So feel free to fire them on occasion.

But without customers, there is no business... it's just a hobby.

Next comes employees because they're the face to the customer and help ensure the customer is satisfied with their service or product.

Last should be the shareholders/silent business partners. If you're doing well with the first 2, there should be sufficient profits to keep the shareholders happy.

This all assumes it's a privately held company. Once you go public, the dynamic changes because you never know who your shareholders might be. Sometimes it's the shareholder's that MAKE themselves the highest priority bc they come with substantial financial and political power to influence the people that actually run the company.