I think this is difficult to answer because the countries where agroforestry is used may not be those were the systems were thought out or developed. France is a country with the early push to develop agroforestry for solving the problems of feeding and maintaining a vastly growing population internationally. The first research on agroforestry was conducted at the University of Montpellier.
The idea was then developed at the heart of the Agropolis scientific community. Large areas in Africa and Asia, in countries such as Niger in Africa, Vietnam in Asia, are benefiting from the insitution of agroforestry practices. I cannot find which countries specifically use the systems most.