There were so many "great" acts by "small" people prior to, during, and after the invasion it is hard to point to just one or even a hundred acts that could be "the" feat of daring or ability that made the invasion a success.
British paras at the Pegasus Bridge, the American Rangers at Point du Hoc, the infantry in the first wave at Omaha Beach....the destroyer men that went right up on the beach delivering direct naval gunfire....how does one single out the one feat above so many others.
Our Fathers and Grand Fathers that participated in that grand adventure all are heroes in my book. They left us a great legacy to live up to.