Some months ago I visited the NS Documentation Center of the City of Cologne. The museum was very interesting for me because the building wasn't destroyed during World War II and I could see the former Gestapo prison with inscriptions and drawings done by prisoners on the walls.
Now I am looking for other museums in Europe dedicated to memorializing the victims of the Nazi regime like prisons and concentration camps where I can see more than just photos and videos, or some memorials built after the war. I want to see the death camps from inside, prison cells or gas chambers.