The Berlin Wall, definitely – for me, anyway.
I just found this awesome website, which maps our 3-, 4-, and 5-day itineraries for visits to Berlin – complete with an app that links to your calendar…http://www.visitacity.com/en/berlin?gclid=CjwKEAjwqZ7GBRC1srKSv9TV_iwSJADKTjaDjGp4Xm3Pj8nj58DebLj-RRsPFJAhGxWSguf1GasrRBoCHAbw_wcB