I don't know about "the worst." That's a value judgment that I don't know if I'm qualified to make. There are some important differences between American slavery (used in the broader sense) and other forms of slavery. Take Ancient Rome, for example, certainly many slaves, but slaves could become free and eventually become citizens, if they saved up enough money. In many regions of Africa, around the fifteenth and sixteenth centuries, slaves were treated kinda like family members. They could engage in the local economy, marry whom they pleased, and even inherit. The Comanche in the eighteenth and nineteenth centuries also kept slaves, frequently Europeans or other natives that they had captured. These slaves could become members of the tribe, either through marriage or through very grueling rituals and requirements.
One of hte main things that distinguishes American slavery is that it was what we refer to as chattel slavery. Basically, being an American slave meant that you were always gonna be a slave unless your master freed you. Furthermore, all of your kids would be slaves, and their kids, and so on. This was not the case with the Comanche or the Africans. American slavery was also pretty brutal. The continental US was the only area where slaves naturally increased in number. In the Caribbean and Brazil, the two main destinations for slaves coming across the Atlantic, the odds of living to old age were very very low.
So, I don't know if one can call a "worst" slave society, but American slavery sure wasn't great.
Sources: Joseph Miller, The Problem of Slavery as History, Philip Curtin, The Rise and Fall of the Plantation Complex, Diouf, Fighting the Slave Trade, Hamalainen, Comanche Empire. (note, I'm reciting these books from memory so I may have misspelled a name or two.)
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