I'm asking for a history project; I have searched online, but have been unable to find an answer.
Portuguese ships carried more slaves to the Americas than any other flag carrier, but the numbers cited include Brazilian carriers
Although the time periods for the two sources cited are different, both include the years when the large majority of slaves were transported
Portugal / Brazil (E) 5.85 million, (D) 5.10 million Note: Brazil only became independent in 1822, by which time the numbers embarked / disembarked were 4.34 million / 3.82 million
Britain (E) 3.26 million, (D) 2.73 million
France (E) 1.65 million, (D) 1.38 million
Spain / Uruguay (E) 1.06 million, (D) 0.88 million
Netherlands (E) 0.55 million, (D) 0.48 million
This (older) source gives different numbers, but the top two flag carriers are the same.
Portugal / Brazil 4.65 million
Britain 2.60 million
Spain / Cuba 1.60 million
France 1.25 million
Netherlands 0.50 million
Wikipedia has a table of Destination of enslaved Africans (1519–1867):
Destination Percentage British mainland North America 3.7% British Leeward Islands 3.2% British Windward Islands and Trinidad (British 1797–1867) 3.8% Jamaica (Spanish 1519–1655, British 1655–1867) 11.2% Barbados (British) 5.1% The Guianas (British, Dutch, French) 4.2% French Windward Islands 3.1% Saint-Domingue (French) 8.2% Spanish mainland North and South America 4.4% Spanish Caribbean islands 8.2% Dutch Caribbean islands 1.3% Northeast Brazil (Portuguese) 9.3% Bahia, Brazil (Portuguese) 10.7% Southeast Brazil (Portuguese) 21.1% Elsewhere in the Americas 1.1% Africa 1.4%
So... Portugal then?