Luxembourg. Because Luxembourg.
All I know about Luxembourg is that they are one of the wealthiest countries, right? Is there something more I should know...
It's small, it's cute, it speaks 4 languages, offers easy access into the 3 to 5 surrounding European countries. It's young, and a relatively vibrant place for its size (lots of people in their mid-20s coming in to work at the EU institutions). And it has a ton of medieval castles.
In truth, you will get a million different responses, because you specify very little about what you're looking for. I spent a year in Luxembourg and felt very comfortable. Visited Paris (multiple times), Bruxelles, Germany, could have gone to the Netherlands and Switzerland if I weren't too lazy.
The center of Europe is not a bad place to be if you dig the urban lifestyle and like to travel. But if you want sun, and palms and giant waves, perhaps not so much