The issue come down to what is pollution? If we are talking about industrial waste that is not treated or neutralized in any way, the third world countries are the worst offenders. India and China would top that list.
China produces more air pollution then any three other countries. The only one where their is any competition is with CO2. This is not considered a pollutant by most science groups, but politically it is a current topic. (politics always trumps science). China produces 27.3% and the USA produces 24.8%. France has cleaned up most of it's CO2 pollution by installing many new nuclear power plants.
This, quite naturally, make France the leader in Nuclear waste, which is not a pollution, but a waste management issue.