I live in a fairly rural area in Germany and even if the nightsky is perfectly clear the cities in the distance cast a bright haze into the sky. They aren't that dark...
Death Valley in California has pristine night skies. I cannot even begin to describe how different it is. Think - fine diamond dust on black velvet. There are stars everywhere you look. Amazing.
The most preserved one is surely around the Pic du Midi in French pyrenees. It is actually an Dark Sky Reserve since 2013.
And according to this map¹, North of Scotland seems a good spot too, among others.
1 : https://avex-asso.org/dossiers/pl/europe-2016/ (in French, but the colors are pretty self-explanatory)