We are in a time where a single invention can rarelt be great. For technological development you need thousands of small inventions, each that use previous technological breakthrough through decades of research. And even great things we have, are just refinement and miniaturization of things we already had.
But if a single thing had to be said, I would say mRNA vaccines. Covid vaccines saved milions of lives, were developed in record times, and their technology could be used for HIV or even antitumoral vaccines.