Uspensky Cathderal
The main Orthodox church in Helsinki, the red-brick Uspensky was built in 1868.  It sits on the waterfront on top of a rocky outcropping.  The outcropping is not all that high, but it's enough that the church dominates you.  It is visually stunning day and night, and is lavishly decorated inside.  I was lucky enough on a cold-but-clear January morning to catch the sunlight streaming in at an extreme angle through the windows, highlighted by dust.  It was noon.