Basically, there was no room for it. In the Second World War for example, between the size (and nature) of the armies, the power (and abundance) of the weaponry, and the political accountability, there was very little scope for individuals and/or small groups to come up with and implement extraordinary wheezes as previous battlefield commanders had. There were isolated and smaller scale examples of it, most notably on both sides in North Africa and the Mediterranean, where half the time they didn't have enough equipment and were given a relatively free hand to make the best of it, but nothing comparable in terms of individual input to something like Frederick the Great taking on the rest of Europe or Gustavus Adolphus tearing through Germany. As far as the twentieth century goes your best bets for military genius would be people fighting insurgent campaigns, and doing so with massive scope for personal command, like Christiaan de Wet and Paul von Lettow-Vorbeck.