If something's not listed, feel free to add and discuss it in the comments.
My vote went to Lou Gehrig. That moment, on July 4th, 1939, transcends not just baseball but sports. It became a movie; it's still referenced today. It wasn't just about a great Yankee first-basemen, it was about a great human being. It was tragic, yet inspiring, uplifting and transcendent at the same time. In so many ways did it define not just an era but a team, and possibly re-define the American pastime.
He (and that speech) may have changed baseball from a game of drinkers, smokers, racists and indulgers to a kinder, more refined game of Joe D, Ted Williams and Jackie Robinson. (The war may have also contributed.)