In an emergency situation that requires quick decisions, a president would most likely use the tools of Executive Order and State of Emergency Declaration.
An executive order is a directive issued by the president that manages operations of the federal government, and may have the force of law. Executive orders allow the president to act quickly and decisively to address issues within their power, without requiring the approval of Congress.
A state of emergency declaration, on the other hand, allows the president to activate emergency powers and resources to respond to a crisis, such as a natural disaster or national security threat. The declaration of a state of emergency grants the president broad powers, such as the authority to deploy federal troops or to allocate federal funds for disaster relief.
The power of pardon, the law of presidential succession, the power of veto, and annexation are all important tools of the presidency, but they may not be as directly applicable in emergency situations as the tools of Executive Order and State of Emergency Declaration.