Left ventricle .
Step-by-step explanation:
Myocardial infarction commonly known as heart attack occurs when there is inadequate supply of blood to the heart.
Since the left ventricle of the heart is much thick when compared to other chambers, its oxygen demand is much more. To fulfill this oxygen demand it needs maximum supply of oxygenated blood.
Thus if the blood supply to heart is lowered due to any reason then the left ventricle will be affected at first becoming the most probable site of myocardial infarction.