If you exclude the heart, aorta is the blood vessel with highest blood throughput. If the heart is included, then it’s probably up to how you measure it – the left heart (or more precisely, the left ventricle) pumps blood to aorta, which in turn leads it to other parts of the body, save the lungs. So the blood volume that passes through the left ventricle is pretty much the same that enters aorta, but the blood flow rate per cm2 may vary depending on the diameter of the aorta, as well as the elastic nature of the artery wall that first expands after the initial heart beat, and then contracts to give its potential energy to the blood flow.

So, to make a long story short, it’s either the aorta or the left ventricle, depending on what values you measure. Someone correct me if I’m wrong 🙂