typically the center of Berlin is considered everything inside the first railway circle, if it is about a point it is usually one of these: Brandenburg Gate, Alexanderplatz tower or maybe Potsdamer Platz, also Friedrichstr./Unter den Linden is a candidate. For Berliners it’s often also Breitscheidplatz/Kudamm (West-Berlin), and generally the city is considered to be polycentric (additionally Hermannplatz and Gesundbrunnen are considered centers).