What are the most efficient heat exchange sites on the human body?
From personal experience, the best places to put a cooler (at maximum metabolic load) are across the back and over the neck and head.

Good candidate areas for a heat sink or exchanger: 

Any region where large volume per unit time of blood travels close to the skin. Some dinosaurs are thought to have cycled blood through spiny fins on their back to cool down (obviously this works for non-dinosaurs too, and even better if the blood vessels in the fin are pressurized (see Are there any known examples of animals with air-conditioner type cooling? ).

Regions where a lot of metabolic activity takes place, including muscles in active use and "brown fat" (see https://www.webmd.com/diet/features/the-truth-about-fat)