My answer within the last few decades would be Alexander Grothendieck and I’d place him higher than Tao, Scholze and any other living (or dead) mathematician within that time frame.

Few in the HN crowd understand the mathematics Grothendieck worked in and therefore cannot properly assess his contributions and intellect due to their unfamiliarity with his work.

von Neumann had savant like intelligence, but Grothendieck was in an unmatched league of his own in terms of abstractions.