Which duty expected of citizens is the most important? why?

If we are talking in terms of America, then voting or following the law. This is because we have many privileges as Americans and following a few laws should be very simple with all the rights we have, and voting because voting in America shows that you're contributing to the country and helping your fellow Americans pick a new leader for our country.

I hope I could help!

Ifound this on google “separation of powers. a fundamental principle of the united states government, whereby powers and responsibilities are divided among the legislative branch, executive branch, and judicial branch.” i hope this you for what your looking for