What do you think was the greatest scientific advance of the hellenistic period? why?

It was medicine.  During this period Herophilus of Chalcedon made many discoveries in this field.  These include a detailed description of the brain as well as identifying each part’s function, the importance of a pulse in diagnosing illnesses and that discovering that arties only contain blood.  Erasistratus was able to discover the valves of the heart and differentiate between motor and sensory nerves.  He also the connection of veins and arties and was against excessive blood-letting to cure people.  Medicine is the most is the most important because the advancement made during the Hellenistic period or any period that strove to find better ways to treat those who were ill or injured.