Unless you are capable of time travel you will never see Hitler’s Bunker. The Red Army destroyed it in 1947. An information board on the original location at Gertrud-Kolmar-Straße corner In den Ministergärten gives you the best impression of the size and its history.
However, there are still some WWII fortresses left to admire. Since Berlin is built on a swamp most bunkers are Hochbunkers, meaning overhead hoppers. This makes them not only spectacular but also better photo opportunities.
Here are some of my favs.
1) Reichsbahnbunker Friedrichstraße
Built in 1943. After the war it was used as a prison by the notorious NKDW, later as storage for textiles and tropical fruits from Cuba. Between 1992 and 1995 this was the HQ of Berlin’s gabber scene.
2003 the Boros Collection (contemporary art collection) moved in and made it open to public. Highly recommended! Book well in advance http://www.sammlung-boros.de/besuch/info.html
2) Hochbunker Pallasstrasse
What a beauty. This hulk was only finished on the 1980 and is still used as a bunker nowadays.This one, as most, was built by forced labour from occupied countries. US army tried to bring this bunker down with plenty of dynamite after the end of the war, however they did not succeed. 1977 this ghastly building was accompanied with another beast – 10 story concrete prefab on stills. A must see for urbanism junkies.
3) Flakturm Humboldhain (above-ground, anti-aircraft gun blockhouse towers)
Climbing on top of the remains of this flak tower gives you a breathtaking view of north Berlin. You can also arrange a visit to the adjoined cold war bunker complex with the guides of berliner-unterwelten.de who are more than happy to introduce you to the horrors of everyday war life. http://berliner-unterwelten.de/home.1.1.html