Probably Portugal was the most successful slave trader because they may have been the first country to utilise the practice, or at least the first on record. It was they who set up the sugar plantations. Sugar had been unknown in Europe and it was greatly prized.
This appetite was recognized, the plantations extended and with them, the growth of slavery. Slaves were shipped to South America - and this part of the world developed as a result. A century or more later, the Dutch got into the slavery act. The British followed. Great riches were made on the backs of slave labour.