Which branch of government at the national level is most important, in your opinion?

This depends on what you mean by "important."  In my opinion, the executive branch is the most important branch of government at this level.

I believe that this branch is the most important because it is the branch that includes the president.  While the president, of course, has no power to make actual laws, I would argue that he (and eventually she) is the most important political figure in the country.

It is the president who has the "bully pulpit" that allows him to set the agenda for the country.  It is to the president that people look in times of crisis.  So even thought the president doesn't make the laws, he has much more influence on what people think of the government and on what sorts of things the government does than Congress typically does.

So in that sense, I think that the executive branch is most important.