mRNA vaccines takes the first place for sure, and by a wide margin. But also advancements in lithium ion batteries and solar cell technology. We also finally generated electricity with fusion power this century, traveled to Pluto and set up the James Webb Space telescope [albeit the latter two are engineering achievements].

I think using an absurd amount of power to train a neural network on stolen data which is then used to perform mundane tasks and performs significantly worse then if done manually. If anything LLMs is one of the least impressive, yet most annoying technological achievement this century.