The first, last and largest player in the modern slave trade was Portugal. Over 40% of Africans were moved by Portugese, mostly to Brazil.
-----------------------------------------------------------------------------------------------
Trans-Atlantic imports by region
1450-1900
Region                              Number of slaves      accounted for %
Brazil                                          4,000,000          35.4
Spanish Empire                           2,500,000                  22.1
British West Indies                           2,000,000                  17.7
French West Indies                     1,600,000                  14.1
British North America
   (and United States)                       500,000                    4.4
Dutch West Indies                              500,000                    4.4
Danish West Indies                        28,000                    0.2
Europe (and Islands)                       200,000                    1.8
-----------------------------------------------------------------------------------------------
Total:                                     11,328,000                 100.0%

Here is the article:

          Trans-Atlantic Slave Trade