Even on a tight schedule, Stasi Prison Memorial (Hohenschönhausen) earns the hours.
Allow 2 hr (incl. travel).
Being led through the cells and interrogation rooms by someone who was actually imprisoned here is one of Berlin's most powerful, unforgettable experiences. The catch: it's far out in Hohenschönhausen, only really sensible with a (timed) guided tour, and is emotionally intense.