Determining which nations are the strongest in Judo is a complicated calculation. In this article we shall approach this by looking at Rio2016 Olympic games.
Technology makes investigating this subject somewhat easier than in earlier times. The IJF operates the http://judobase.org website, which includes all data from IJF events since 2009, including the Olympic Games in Rio.
From these two metrics we can easily identify that Japan tops both lists, France and Italy appear on both lists as well. USA does not appear on the second list, but only just they are 6th on that list.
These two metrics suggest that Japan and France are the strongest nations.
Other metrics need to be considered also, for example number of athletes qualified for the event (excluding the home nation Brazil that had automatic qualification):
Again, France and Japan top the list, which could infer depth of talent. France and Japan were the only two nations that had 14 athletes compete in Rio.
More investigation is required clearly, but even this simple examination can give some information and importantly raise questions around the topic.