The most common cause of Addison's disease is -
Most common cause (60%-70%) of primary adrenocorticol insufficiency (Addison'discases) is autoimmune adrenelitis.
Other common cause are.

Tuberculosis → Most common cause of Addison's disease in India.
AIDS
Metastatic cancers
Autoimmune adrenalitis
Meningococcal septicemia
Malignancy
Tuberculosis