The building of the German Federal Environmental Agency – built in Dessau and opened for buisiness in 2005. I was genuinely impressed. It’s an administrative building after all – and they tend to be a bit dull and boring. But this one is really beautiful. It is located in Dessau, a small town in the eastern part of Germany (roughly 50 km south-east of Berlin to be precise), which was also the location of the famous Bauhaus school of architecture. I guess that was a bit of an obligation as well as an inspiration.
On a historical note: One of the teachers at the Bauhaus school was Lyonel Feininger, father to Andreas Feininger. You might have heard of him… 
Thomas