Having been to Europe a few times, specifically the U.K.(London, Cardiff, etc.), France and Germany I have seen some very nice Church structures.
However, I still think the Shrine of the immaculate conception right here in D.C. on the campus of CUA is amazing. I did a photo assignment there back in the early 2000s and took some amazing photos.
Another place is the Mormon Temple, which you can see from the capital beltway near Wheaton, Maryland. Never been inside, but been on the grounds.