Duomo di Sienna, Italy - OK, so I'm a total sucker for a good ceiling. And this is a very, very good ceiling. Seriously though - give me a gorgeous Gothic ribbed vault over the shiniest skyscraper in London, any day. It ages better, and it's an equal if not greater marvel of engineering.
45 - Duomo (Siena Cathedral) - Siena, Italy by Batistini Gaston. Truly one of the most beautiful cathedrals I have ever visited!