According to Zuckerman, Galen and Pasquale, in their super book The Nonreligious, the least religious countries in the world are:
Czech Republic, Estonia, Sweden, Denmark, Norway, Finland, China, South Korea, France, Vietnam, Russia, Bulgaria, Japan, the Netherlands, Slovenia, Germany, Hungary, Great Britain, New Zealand, Canada, Australia, Belgium and Latvia.
This is derived from ideas of separation of church and state, self-identification of secular and atheist/agnostic qualities, low level of religious belief, participation and observance.
With the exceptions of China and Vietnam (both nondemocratic nations), the majority of the most secular countries in the world today are actually doing quite well. Indeed, many of them are among the most successful, healthy societies on earth. For it is generally among the most secular societies today that we find the greatest levels of social harmony, civility, freedom, equality, peacefulness, and prosperity. The greatest levels of inequality, oppression, crime, corruption, and destitution can be found in highly religious nations.
On virtually every level of well-being, the more secular, the better. Now, obviously, there are many variables at play here, and unpicking those variables is the point of the book in this section, but as a conversation starter here, it is worth dispelling the myth that religion is necessary for a properly functioning society. Gingrich, Hannity, O’Reilly and others can take a running jump on that one.