This was an interesting article. Check out the map to see what is the top site or thing to see in every country of the world. They often are not what you think they would be.
http://www.huffingtonpost.com/entry/what-to-do-in-europe_us_58d2cecae4b02d33b74806e6
I can't even think of reducing sites to just one for each country. Can you?