The most stable border in Europe is the one between Germany and the Netherlands. It has been the same for 800 years, except 1807-10, when Napoleon incorporated Ostfriesland into the Kingdom of Holland, and except for some very small annexations that the Netherlands did after WWII. Furthermore, most of those annexations were annihilated in the 1960's, so that the border has only moved a few hundred meters on a couple of places.