Standing on the spot where the trains were unloaded at Birkenau
If you are ever visiting Auschwitz, though, I cannot recommend enough that you do not take a tour and spend a decent amount of time at Auschwitz-Birkenau as well as Auschwitz One. Birkenau was the main camp, and while it is not as user friendly as the other camp and just over 3 km away, it was here that most people mean when they talk about Auschwitz. Tane and I spent a couple of hours wandering alone around Birkenau's grounds (for some reason most tours don't seem to come to this camp except to look at the watchtower by the main gate), which was a very haunting experience.
Schindler's Factory in Krakow, which we visited this morning as well as seeing lots of other scenes from the movie Schindler's List.