The valve between the left atrium and left ventricle is the

The answer is the bicuspid valves. 

Bicuspid valves are the valves with 2 flaps, where the main purpose is to prevent blood in the left ventricle from flowing back to the atrium.

This is particularly important because the blood from the left ventricle is going to flow through the aorta, which transports blood to all the other organs (except lungs). If the blood from the left ventricle flowed back to the left atrium, not only this will affect the other blood from flowing to the left atrium from the pulmonary vein, it will also affect the functions of other organs. If the organs don't receive enough oxygen and nutrient from the blood, they may even die. 

So, the answer should be bicuspid valves.