Which country has the most effective democratic system of governance?

None.

Each country has flaws within its system. Flaws found in one, may not be found in another; but, in that other country, there will be other/different flaws. Effectiveness in different areas, varies according to the degree of and number of flaws.

Explore Sweden's form

http://www.sweden.se/eng/Home/Society/Government-politics/Facts/Swedish-System-of-Government/

This does mean every Swede is content and think everything is perfect in Sweden - also here are flaws.

Should be
This does not mean every Swede is content.........

You're just focusing on the national government's selection of a chief executive officer, and that's misleading. In the United States, citizens vote every year--elections for the House of Representatives and the Senate take place every other year, and for President, Vice President, the House of Representatives and the Senate every fourth year.

But Americans vote (or have the opportunity to vote) every year. What do they vote on? They elect judges, they elect municipal, country and state officials. They vote on ballot initiatives which may change the law. They vote on tax levies and bond issues, both of which affect, directly or indirectly, their own finances. They vote for school board officials. They vote for the county Sheriff, and sometimes other law enforcement officials.

I'm not comparing this to other nations, because i don't know how local level governance and taxation is managed in those other nations. However, to judge how democratic the United States is solely on the national election cycle each four years would be very misleading.

North Korea

probably the Nordic countries