As an architecture student, I'd vote for Berlin. It is a who's who of 21 Century architecture (don't mention zeee war!).
A brief list of must see buildings are:
Neues Museum - Chipperfield [simply stunning restoration, the most sensitive approach to an existing building I have ever seen]
Altes Museum - Schinkel
Dutch Embassy - OMA [you will need to book in advance]
Deutches Histories Museum - I.M. Pei [ real nice extension]
Tiergarten Cementary - Axel Schultes [ a bit of a journey but undoubtedly worth it, second only to Chipperfields Neues for me].
GSW Headquarters - Sauerbruch Hutton [a much photographed exterior].
Holocaust Memorial - Peter Eisenman [surprisingly emotive for Eisenman]
Jacob-und-Wilhelm-Grimm-Zentrum (Library) - Max Dudler [ very rational exterior ]
Jewish Museum -Daniel Libeskind [ personally it is a shocking response with Libeskinds ego getting in the way of the exhibits but either way it will provoke reactions. A stunning installation of the 'lost souls' is the finest installation I have ever seen]
Social Housing - Herman Hertzberger [ a fine example of social housing very close to the Jewish Museum]
Academy of Arts - Behnisch + Partners
The Nordic Embassys - [various] worth a look but you wont get inside.
Reichstag restoration - Foster & Partners [ still not been inside as the que was ridiculous both by day and night but I cannot wait to get a fisheye shot of the roof]
Berlin Philharmonic Hall - Hans Scharoun [You simply must attend a performance. You will be blown away.]
Neue Staatsbibliothek - Hans Scharoun [worth a pop in if your nearby the philharmonic]
On top of those you have Gehry, Koolhaas, Karl Friedrich Schinkel, Michael Wilford, Renzo Piano, Jean Nouvel, Gropius, Niemeyer,
Im sure I will think of more but some to get you started.
