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.
Most of the reforms that you mention here had the effect of changing relatively minor things about government. The recall, for example, is rarely used. The initiative is very important, but only in a few states. It does not greatly change the nature of democracy for the country as a whole. The direct election of senators is quite important, but does not reach the revolutionary importance of giving women the vote.
Giving women the vote was important for at least two reasons. First, it made America a more just society. It was simply wrong for us to deny women the vote and rectifying that mistake was one of the most important things that could have happened in American history. Second, it has helped to change American society. When women got the vote, they moved along the path to greater equality and a more important role in society. Enfranchising women helped to change our society and to bring about the more nearly equal society that we now have. For this reason, I see the 19th Amendment as the most important of these reforms.
It's tough to identify just one amendment or change that I would consider the most important of the Progressive movement, but if I had to pick one, I would say bringing about direct primaries. I think allowing the people, rather than the party, select the candidate best symbolizes what the era was about.
When I think of the Progressive movement, I think of a time where government was being reformed, and corruption in high places was being weeded out. With direct primaries, the control of those who had money or significant political pull was diminished, and in place the common folk had a greater say. Though the effects of direct primaries were not fully realized, especially on the federal stage, for years, direct primaries paved the way for a candidate to be representative of the people, rather than the political machine.
As an example of why direct primaries are significant, I want to point to the 1912 US Election. The Republican primary pitted incumbent William Taft against former President Theodore Roosevelt. Only thirteen states actually had primaries that allowed citizens to vote for their preferred candidate, and Roosevelt was overwhelmingly favored over Taft. However, as the current President, Taft had the backing of many political organizations, thus allowing Taft to get the Republican nomination, despite his unpopularity. This is something that wouldn't be able to happen in politics today, and if direct primaries were fully implemented in 1912, Roosevelt wins the nominee, and probably the presidency.
As a side note, the most important action of the ones you listed above may very well be the 19th amendment, permitting women to vote. However, the reason I didn't state that as what I feel is the most important event is due to the fact that I consider it more of a human rights victory, than a political one that would change government and weed corruption. Reform in government was something that was a huge rallying cry in during the movement, whereas the fight for womens' suffrage was something that had lasted before that. In addition, the 19th amendment was enacted in 1920, right at the end of the Progressive Era.