The Alexander Nevsky Cathedral in down town Sofia is one of the largest orthodox cathedrals in the world. If you pick up any book on Bulgaria, you are certain to find a picture of this building inside (and quite possibly on the cover as well). It is the cathedral of the patriarch of the Bulgarian Orthodox church.
This building is beautiful from any angle. The golden domes can be seen glistening in the sun while hiking on nearby Vitosha mountain. If you come to Sofia, and miss out on Nevsky Cathedral, you have missed out on Sofia.