This depends what you class as "boring". Northern Europe is much more easy going, not as cultural, whereas in the south (Spain for example) are more cultural, which could eliminate the "boring" factor. I've traveled around Europe, to many places and from my experience, the country that I found the most "boring" would probably be Belgium. There's not much there, obviously you have the cities, but they don't really have an atmosphere unlike say, Rome. It depends entirely on opinion, and what you class as "boring". No European country is "boring" in a sense. There are many things in each of them.
The United Kingdom
Belgium
My top 5 boring - Belarus, former yugoslavia,Austria, slovakia,sweden
England and Holland
Dirty and boring?Without second Croatia and Albania and Kosovo
I guess Belarus and former Yugoslavia.
poland