Final answer:
The most difficult FDA approval classification to obtain is Class III, which concerns devices that are vital for sustaining life or preventing significant health impairment, and require rigorous testing and clinical trials.
Step-by-step explanation:
The hardest classification to get FDA Approval for is Class III. Class III devices are those that support or sustain human life, are of substantial importance in preventing impairment of human health, or which present a potential, unreasonable risk of illness or injury. Examples of Class III devices include implantable pacemakers and heart valves. These devices undergo a rigorous premarket approval process (PMA), which is the FDA's most stringent review. They must be proven to be safe and effective before they can be marketed, and typically they require clinical trials.