Kotnice Ossuary in Kutná Hora, Czech Republic. The bodies are from Black Plague and Hussite Wars of the 14th and 15th century. The chandelier in the middle of the Ossuary might just be the most interesting piece. It makes use of every bone in the human body.
This scene can't help but remind me of the Catacombs in Paris I visited a year and a half ago.
Seeing stacks of thousands of people's bones is eye opening and quite sobering to the idea of how precious life is and unavoidable fate of death.