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• There are many parties that make up a firm. Class example: you can not go and hit a firm. Who can you hit?
• Shareholders, bondholders, employees, customers, management, suppliers, etc. All of these are called stakeholders. Much debate as to which group deserves most attention. 
• This stakeholder controversy is especially severe during control activities (example acquisitions). This has led some states (Pennsylvania in particular) to allow the Board of Directors to consider all stakeholder groups when deciding on a merger.

The short answer is that stockholders are the most important, but we often read or hear reports that look different from this claim. Many authors and business leaders cite differing stakeholders as the most important. For example in Hope is Not a Method, Gordon Sullivan writes:

In the final analysis, everything comes back to people. People are not in an organization, people are the organization. The bricks and mortar, machines and computers are there only to leverage the power of these people….it is the people in your organization that make the difference.

Want further evidence?  Many marketing oriented books and businesses, the customer is deemed to be the most important. For example at Stew Leonard's grocery stores there are three-ton rocks stating the rules of the company. 

Rule #1-The customer is always right. 

Rule #2 is that "If the customer is ever wrong, reread Rule 1." 

While we can question Leonard's ethics and tax-accounting (but that is another question), we cannot argue with the success of his organization nor the world-wide acclaim that he has won in serving his customers.

Why the disagreement? In part it is because the various groups all want to be the most important. For example, management frequently argue that they make the decisions and thus are the keys to the organization and as such are the most important stakeholders. Why does Stew Leonard feel that customers are the most important? Why do Southwest Air and Gordon Sullivan think employees are the most important?

Probably a more likely explanation is that the argument centers on semantics. If pushed most if not all of these stakeholder advocates would likely agree with Financial Theory (and common sense) which holds that stockholders are the most important group.

• Without an emphasis on employees or customers the goals of the shareholders will not be satisfied. In Turned ON Roger Dow and Susan Cook describe this by stating that the employees and other stakeholders form the foundation upon which financial goals can be satisfied.   Stop and picture that.  The foundation is that which all else is build upon. So without employees the entire nexus falls apart.

That mental picture is an effective tool to keep in mind when out in the business world. 

If employees are not satisfied, they will not put forth the effort needed to satisfy customers which will result in poor customer satisfaction and subsequently poor financial performance and poor stockholder returns. Thus the argument over which group is most important is really an argument as to how we should focus our efforts in an attempt to satisfy the most important group--the stockholders.

• They are the owners
• They have taken the risks 
• As residual claimants they are in the best position (have best incentives) to monitor the firm. Further, as residual claimants maximizing their returns in a world of efficient markets and available information leads to all other claims also being satisfied. Thus the implicit and explicit contracts that make up the "nexus of contracts" have all been satisfied.

For example: Bondholders and shareholders fight over many things (for example risk) while Shareholders and management disagree over risks, managerial effort, timing of cashflows, etc. (this shareholder-manager conflict is known as the Principle Agent problem and is an example of an agency cost-see Jensen-Meckling 1976 or Adam Smith 1776)

These "fights", or at least the potential to fight, between stakeholder groups have shaped the financial arena. They influence the types of securities that firms issue, the amount of debt firms have, the compensation structure of management (and employees), the governance structure of firms (for example the Board of Directors is designed to look out for the interests of shareholders), as well as the actual terms of contracts.