Great britain and france responded to germany annexing austria and the sudetenland

What would be considered as the most important reform of the Progressive movement and why?

I have to choose from: 16th, 17th, 18th and 19th amendments and direct primaries, initiatives, referendums or recall.  

This is, of course, a matter of opinion.  My own view is that, of the reforms listed here, the 19th Amendment is the most important.  This is because it did the most to right a wrong and has arguably had the greatest impact on American society.