It is really hard to tell which country has been the most affected, since the traffic is illegal, and largely invisible to the public. I would have to say that Mexico is one of those most affected though. There is a vast amount of drugs, legal goods and humans that cross the border from Mexico into the United States every day, and along with that traffic is, tragically and horribly, slave trafficking as well.
Most of the victims are children and teenagers, and are sold into sexual slavery inside the United States.
There are other countries where slave laborers are still sold, including in and between some African countries. Follow the links below to the United Nations site on human trafficking and domestic slavery.