Mine is this recently acquired single page of sporting news from the New York Evening Post dated June 14, 1923. The day before Lou Gehrig made his Major League Debut...
1923GehrigJoinsYankesNEWSPAPER.jpg
The reason it's the most important piece in my collection is that it makes my original TYPE 1 photo the first known image of Gehrig in a Yankees uniform as he made his debut the very next day!
From Wikipedia... "Gehrig joined the New York Yankees midway through the 1923 season and made his debut on June 15, 1923, as a pinch hitter. In his first two seasons, he saw limited playing time, mostly as a pinch hitter — he played in only 23 games and was not on the Yankees' 1923 World Series roster."
1923GehrigJoinsYankesNEWSPAPERcloseUP.jpgGehrigYEE2010.jpg
GehrigYEE2010verso.jpgGehrigYEE2010loa.jpg
So what is your most important piece?