My suggestion for the most important invention of the last two millennia is the computer because of the way it extends the capacities of the human mind for accurately performing large numbers of calculations and for keeping track of and accessing vast bodies of data. As with any great invention, these enhanced abilities have a light and a dark side. As a scientist I am now able to answer questions that could not be answered prior to the computer. On the dark side is the loss of privacy and the enhanced potential for social control made possible by the ability to manipulate large databases of personal information.
As another candidate, my mother has said that her all time favorite invention is the telephone because of how it allows her to stay in intimate and immediate contact with distant friends.