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.
Was going to say that too. Regardless of the motives and driving forces behind the incredible speed at which the vaccines were developed (i.e. certainly a similar urgency could be applied to other diseases killing thousands and millions in poorer countries, but there ain't as much interest in that), the mRNA technology proved quite powerful and an avenue to continue exploring in future research.
People forget that the research behind those vaccines had been going on for 30+ years. What was accelerated was the trials and the gathering and analysis of efficacy and safety data. The actual vaccine technology had been in existence for around a decade at the time.
The first successful transfection of designed mRNA packaged within a liposomal nanoparticle into a cell was published in 1989. "Naked" (or unprotected) lab-made mRNA was injected a year later into the muscle of mice.
But on the other hand, first human test was in 2001