A 58-year-old man with a history of hypertension and coronary artery disease presents to the emergency department with palpitations and dyspnea. ECG shows atrial fibrillation with a ventricular rate of 145 bpm. Blood pressure is 110/70 mmHg, and he is haemodynamically stable. Troponin is negative. He has no prior antiarrhythmic therapy. Which of the following is the most appropriate first-line pharmacological agent for rate control in this patient?