What crop that first made slavery profitable?

Slavery made all crops MORE profitable. If you're talking about America, slaves were brought to the Caribbean to cut sugar cane in droves. This is probably the biggest crop using most slaves initially. Cotton became a big slave labor driven industry in the 19th century for several reasons. However, slaves were initially and primarily owned and used as domestic helpers in the Americas. Only once certain industrial processes were invented and began to be applied to agricultural products such as cotton, which allowed its large scale PROCESSING, did the demand for the RAW cotton skyrocket and thus the demand for slaves to pick it increase as well.

If I may say so, the question is backwards. Certain crops did not make slavery profitable, there had always been a market for slaves. It was Slavery which made certain crops profitable.