My immediate reaction to the question was to choose between the printing press and any of a set of public health-related inventions (antibiotics, sewage treatment, ...). And since it seems as though we might never have had the public health advances without the printing press, but did, in fact, have the printing press without the public health advances, I'd have to choose the printing press.
Why? Because it opened the possibility that knowledge (information, wisdom) could be disseminated beyond a small number of privileged individuals, thus permitting larger numbers to share or debate world-views and to build upon past and present ideas. Thus far, at least, new electronic technologies (radio, movies, television, computers) have been employed as extensions of this broadening of access to knowledge, altering the medium of exchange but not the concept. At some time in the future they may lead to more fundamental changes in the human condition, but not yet, I'm afraid.