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.