Mies van der Rohe at the Museum of Modern Art. (Somehow, Flash is less offensive when used to present 'nice' things which you can browse in leisure. This one may be a bit busy, though.)
The Neue Nationalgalerie, shown as the last exhibit of van der Rohe's work, is quite a breathtaking space. The idea to not clutter that space and put all the exhibition halls under-ground was simply genius.
mpt in case you read this – your mailbox quota has been exceeded.