When these periodic "let's put a woman on a bill" come up Noether is my choice for the subject. Her work had a huge influence on the shape of the world today and few people outside physics and mathematics have heard of her.
And she's dead and has a name, which is good enough for the Department of the Treasury. That's the only legal restriction I could find on who may appear on US currency and securities [1]:
United States currency has the inscription “In God We Trust” in a place the Secretary decides is appropriate. Only the portrait of a deceased individual may appear on United States currency and securities. The name of the individual shall be inscribed below the portrait.
See the FAQ [2] at the Bureau of Engraving and Printing site for some interesting information on how the current choices came about.
