The groups that benefited the most from the changes caused by the Black Death were peasants and laborers. These were the people who saw demand for their services grow more than any others.
Before the Black Death, Europe was overpopulated. There was a huge pool of labor available so peasants and laborers had no leverage. They had to remain subservient and obedient because they could always be replaced.
After the Black Death, the demand for their labor was greater than its supply. This meant that these groups had power. They could force employers and landholders to pay them better wages and/or give them better terms. This enabled some to rise up the social and economic ladder.