Sir 20 yr male facing problem of spondlytis and having severe pain in both legs. Pls suggest best medicine for his pain.

Do you mean sciatica. For a 20 yo with back pain, first you must make sure it is not something more serious. As long as there is no associated weakness, loss of bladder control, or evidence of a spinal infection, or tumor, it can be treated with over the counter anti-inflammatory medications. If there is no improvement despite medications, stretching and ice, please see a doctor.
Spondylitis. Depends upon severity. You need a good exam and imaging. Typically the best medicine is one to reduce inflammation.