The answer depends on your personal interests. I have traveled frequently to Italy, France, Denmark and Great Britain. Each country offers a plethora of sights that appeal to different tastes. Italy is my favorite. Foodies can experience the amazing differences in cuisine from risotto served in the colder northern climes to pasta, served up in the warm, sunny climate of the south. Art connoisseurs will relish the breathtaking works of Michelangelo in Florence and Rome. Historians enjoy visiting the Colosseum, the catacombs, the restored cities of Pompeii and Herculaneum. Naturalists revel in the awesome vistas along the Amalfi Coast and Cinque Terre. Travel over to the Adriatic side and you find Alberobello ..
... a small town and comune in the province of Bari, in Puglia, Italy. It has about 11,000 inhabitants and is famous for its unique trulli constructions. The Trulli of Alberobello are part of the UNESCO World Heritage sites list since 1996.
Here is a tip that applies regardless of where you travel in Europe. August is when most europeans go on vacation. As a result, popular tourist attractions offer limited access. It is also helpful if you or another member of the group, has some foreign language training. No need to be fluent - just making an effort to speak their language will go a long way. Happy planning and enjoy the trip!