The Romanian Athenee.
This place is a jewel. Built in 1888, it is the symbol of the new Romanian cultural identity (Romania had become independent from the Ottoman Empire in 1877).
It is also the place with the best acoustics in town, so the most suitable place if somebody wants to listen to classical music. The Philharmonic Orchestra - which has its residence in the Athenee - is, perhaps, the best in town, and in the country. Even if the tickets are very expensive, according to Romanian standards, the hall is usually packed.
Not important, but still: last time I've been to the Athenee, I listened to a very nice Daphnis and Chloe, composed by Maurice Ravel and conducted by a French conductor. It was tops.