There isn't going to be any one answer to this, since slaves came from the whole Atlantic coast of Africa. In the Kongo region, which was the greatest exporter to America, relatively simple loincloths and wrappers would have been fairly common given the tropical heat, though often combined with some kind of cloak or mantle. Among elites (who were quite often enslaved by the 18th century) these might be intricately woven from raffia palm, which impressed one 16th century Portuguese visitor enough to write that ‘In this kingdom of Kongo they make palm cloth with velvet-like pile, and with designs as velvet satin which are so beautiful that better workmanship is not found in Italy', or they might be made from colourful imported European and Indian textiles. Quite often these indigenous and imported materials were combined in colourful ensembles that showed off both one's local status and international connections. Capes and woven or beaded hats were common signifiers of elite status.

Cécile Fromont (2018), Common Threads: Cloth, Colour, and the Slave Trade in Early Modern Kongo and Angola, Art History, Vol. 41, No. 5

Cécile Fromont (2017), Foreign Cloth, Local Habits: Clothing, Regalia, and the Art of Conversion in the Early Modern Kingdom of Kongo, Anais do Museu Paulista: História e Cultura Material, Vo. 25, No. 2. https://dx.doi.org/10.1590/1982-02672017v25n02d01-2

http://www.comune.torino.it/cultura/biblioteche/iniziative_mostre/mostre/missione/index.html