Which of these is the most important duty or responsibility an individual has in a democratic society?

I would say that voting is a very important thing that members of a democratic government but also lobbying for a more fair voting system than first past the post and also being involved in exercising one's freedom of speech to oppose government policies that are prejudicial to the environment, for example.

Vote--democracy is a government type which allows for the voice of the people. In a democracy the voice of the people are allowed.

In a large democracy, voting is the means to get voices heard on an issue or through the support of a political party. For a citizen, voting is exercising the right to have your voice heard. The power of a vote can determine laws pass and people who will represent a person in larger government bodies such as Congress.