I'm not at all surprised that Switzerland is #1, nor that the Scandinavian countries rank high. While speculative, as I've never been there, I would have guessed that Japan and the Netherlands would have ranked higher. I also would have guessed that New Zealand, Canada, and Australia would be more closely paired to the UK. Based on impressions of friends who have traveled and even lived there I was surprised that France ranked as high as they did.
When I was in Paris, I did not find it particularly dirty at all. Paris gets a bad rap.
And Germany, while it is indeed clean, is not at the level of OCD-like cleanliness that is in the popular perception. I dodged the occasional dog turd on the sidewalk in Aachen, something which, contrary to the stereotype, I did
not encounter in Paris. Maybe people kept their dogs at home that day.