The longest surviving part of the Berlin Wall is at the Topographie des Terrors between Potsdamer Platz and Friedrichstraße. (The longer East Side Gallery is on an inner wall part, not the outer wall facing West Berlin as here.)
Being in Berlin, the geography and politics are complicated. To the north (left) of the Wall was East, to the south (right) was West but not directly. The first few meters to the south was still in the East – West Berlin only started somewhere above the glass covers now protecting the walls and foundations of the former Gestapo head quarters.