A 60-year-old woman has heart failure due to non-ischemic cardiomyopathy. Her ejection fraction is 40%, She currently has mild shortness of breath with moderate exertion but no orthopnea or lightheadedness. She has hypertension treated with hydrochlorothiazide and Ramipril. On examination, BP 130/80 mmHg and HR 88/min. Jugular veins are not distended, and lungs are clear, Cardiac examination discloses a regular rate and rhythm with no S3 or murmurs. There is no peripheral edema. Which of the following is the best single best agent to be added to her regimen?