Kazan Cathedral, St. Petersburg 5

This is the most beautiful cathedral in Saint Petersburg in my opinion. It's very monumental and has incredible architecture, really gorgeous decoration of Nevsky prospect (the central street in Saint Petersburg).
The interior is incredibly rich: lots of gold everywhere (seriously!!), Russian Orthodox icons(each of them is worth a fortune). Lots of candles, beautifully dressed priests..., everything there just creates magical and, I would say, even mystic atmosphere.
It should be in top 10 list of where to go in Saint Petersburg for each and every tourist.