Genuinely curious, because I know von Neumann was an intellectual giant, but what was his most original/groundbreaking contribution? My impression of his work has always been that he would survey a field, find some particular problem that needed attending to, tidily solve it, then move on to some other domain. But I guess there's a latent assumption there that he didn't do anything huge or groundbreaking.

"Most" involves great subjectivity, but how about laying the mathematical foundations for quantum mechanics?