A 28-year-old woman from Delhi presents with a 3-month history of palpitations, heat intolerance, and weight loss despite good appetite. On examination, she has a diffuse goiter, tremor, and thyroid bruit. TSH is <0.01 mIU/L, free T4 is 45 pmol/L (normal 10–20), and free T3 is 12 pmol/L (normal 3.5–8). She is started on propranolol 40 mg BD and antithyroid therapy. Which antithyroid drug would be the most appropriate first-line choice, and why?