I tend to put these places into before and after I was born.
Auschwitz/Birkenau - visited in 2009, first time I'd been abroad on my own.
Lots of Germans and Jews visiting (understandably). This was before you needed to book a tour, I just turned up, bought a guide book and wandered round. It didnt really hit me till I got to Birkenau - the sheer size of the place.
Also been to Dachau.
Poznan cemetery, where some of the great escape 50 are buried.
Treptow park, Berlin. Huge Russian cemetery
Now the more recent stuff
House of terror in Budapest
KGB prisons in Riga, Tallinn, Vilnuis, Warsaw.
Stasi HQ and prison in Berlin (shown round by a former prisoner)
I found these places harder to understand as when they closed I'd already left school and was working.
Also the tunnel museum and the various mass graveyards dotted around Sarajevo. They had only been buried
10-15 years prior to my visit.