He was overshadowed in his time by Einstein, but everyone alive at the time agreed that von Neumann was easily the most intelligent human around, possibly ever.
His work was so prevalent and touched on so many fields that he was an equal peer to the leading minds in practically every field of science and was peerless among mathematicians. His work continues to be used to this day in making modern computers operate, despite having lived 100 years ago.
Yet now when we talk about the great geniuses known to most people, he’s rarely mentioned. He was the Euler of his time.