I think it is very difficult to say because there are many of them. But I have seen La Fontana di Trevi which I think is perfect. I would like to be there sometime. You can also find pretty museums and castles in Europe. Another place I would ike to vist is the Eiffel Tower.
I like medieval architecture, so I would say San Martin de Fromista in Palencia (Spain) or Santa Maria del Naranco in Oviedo (Spain). Another candidate is the Castle of Loarre in Navarre (Spain). But it's very difficult to choose one, because there are soooo many wonderful buildings also in Portugal, France, and Italy. All over Europe, really.