I am submitting the 1861 #65. Not the million dollar #82 or some exotic pink #64, but the everyman stamp, the CV $3 number 65. And while I know the A25 design is one of Ron's favorites, it's not to suck up to him that I nominate this stamp.

This stamp is exemplary of the industrialization of the United States. According to the Mystic catalog, one billion seven hundred eighty two million copies of this stamp were produced. The number boggles the mind. They could produce more than a billion of anything in the middle of the nineteenth century? Wow! And because of its billions, this stamp is accessible by all who are interested. Liberty, justice, and a tiny portrait of Washington for all. For less than a Lincoln anybody can have a 150 year old stamp. How cool is that?
And speaking of Lincoln, significantly, this stamp appeared at the beginning of the US Civil War, the defining event of the century and the sadly necessary event in the country's history for hammering out some of the inherent imperfections of the Union. This is the Civil War stamp. If you have one of those poignant letters home from a Union soldier, chances are it is this stamp that is on the cover.
The subject, of course, is George Washington. There is no portrait that appears more often on US postage than Washington's. So uncontroversial a figure of Americana is he that even the Confederates saw fit to put his image on a stamp. (Yes, Jackson also appears on stamps north and south, but he is hardly uncontroversial.)

And the stamp and the design in all it's varieties is like Americans themselves. A spectrum of colors and conditions, everyone different, and all the same.