pusianka

I'm more of a castle guy I guess. If you had to pick one, maybe Mont Saint Michael, just because of its dramatic setting: http://thecontaminated.com/wp-content/uploads/2008/04/mont-saint-michel-1.jpg

Historically the most impressive & important building in the world had to be the Hagia Sofia. Originally dedicated in 360, it was the largest cathedral for over 1000 years, the first mega-dome, and single-handedly changed architectural history. http://www.sacred-destinations.com/turkey/istanbul-hagia-sophia-photos/slides/exterior-cc-Cybjorg-flickr.jpg

Hmmm... There are too many beautiful buildings around, indeed. It also depends on what speaks to you: the building that stuck to my mind the most in recent time was the Pantheon in Rome, because of its simple expressive space (ignore the religious clutter inside).