A 52-year-old man with a history of hypertension and left ventricular hypertrophy presents with symptomatic atrial fibrillation of 3 days' duration. His blood pressure is 145/92 mmHg, heart rate 110 bpm, and echocardiography shows preserved ejection fraction. He has no structural heart disease. Which drug is the preferred first-line agent for rate control in this patient?