Christ the Saviour Cathedral
Orthodox Cathedral in Moscow, Russia. It's actually quite new, it was completely demolished by Stalin and after the fall of communism, it was rebuilt exactly as it was before thanks to private donations (it's said that many of which came from the Russian mafia). It's an impressive structure with a beautiful interior, actually you don't notice how new it is. A dress code is required for entry (no shorts or anything that shows the shoulders), and you have to show respect inside. The Russians have become quite devoted in recent years.