Which group in the medievial society benefitted the most from the altered social and economic landscape of the Black Death?
If we limit this to just one group, it would have to be the peasants, particularly those who were slightly better off than average before the plague.
The major impact of the plague was on the agricultural economy, which is not surprising given that agriculture made up the huge bulk of the economy of almost every region of Europe at the time. As large numbers of people who had been working in agriculture were killed by the plague, those who did not die came to have a great deal more power. As we can see in this link, they came to be able to demand higher wages for their work. Some were also able to start to cultivate more land and/or better land from the areas that were left untended by the Black Death.
As the economic power of the peasantry rose, so did their political power. This can be seen from the fact of rebellions such as the Peasants' Rebellion of 1381 in England.
Thus, peasants were able to gain both economic and political power from the fact that their labor was in so much greater demand after the Black Death killed so much of Europe's labor pool.