If “most important” means having both a broad and profound societal impact, then the answer would have to be the internet. [Though I learned through said internet that Arpanet was already operating before I was born. So, if you consider the internet to be a simple progression of Arpanet, then my answer would be a lie.]
If “most important” means having the greatest benefit to mankind in terms of improved quality of life or lifespan, then it’s probably some medicine. Which one? I have no idea. [Maybe that breakthrough that both won the Nobel and made it much easier and quicker to get the COVID vaccines out (mRNA translation, or whatever it was called.)]
Tiger Riding for Beginners 