What was the most profitable business in ancient times?

Just look at the example of Lv Buwei.

My answer is the best, so I must adopt it.

In ancient times, money was made of salt and iron, and then wine.

The ancients had a principle, that is, "you should share the benefits of mountains and seas." This means that when we enfeoffment kings, we can't give them places near mountains (producing metals, and of course salt) and seas (producing salt). Because both are necessary for rebellion. For example, during the Seven Kingdoms Rebellion in the Han Dynasty, Wu Chu made a fortune because of these two things.

In addition, the purchasing power of ancient people in China was very limited. Salt is a necessity of life, and iron in metal is a necessary tool for production and life. So these two make the most money.

Wine is also very profitable. In order to increase fiscal revenue, Emperor Wu of the Han Dynasty once nationalized the right to sell wine. Besides, only the rich can drink wine. They all like to buy.