mRNA vaccines, for me. I'm sure most people read this fantastic write-up of how the covid vaccine works back in December: https://berthub.eu/articles/posts/reverse-engineering-source...

It really opened my eyes to how far we've come over the years in learning to game our immune system. And the existence of printers for custom DNA blew my mind. I can't even get printers to reliably put ink on paper, let alone create the building blocks of life.