Question

What nations dominated the atlantic slave trade?

Answer 1

It's a leading question, as cross-Atlantic trips carrying slaves would be European of all kinds. Though vikings usually enslaved people from the Baltic area but had slaves (thralls, in Danish; "trælle") from just about everywhere.
The Dutch and Belgians were far nastier than most nords, as at some point the Spanish and the Germans. Not to mention the old empire of the queen.