I like this one:

http://www.mnad.org/

The furnishings are fabulous and it lets you see how the really rich lived in BA in the first half of the last century.

The other similar experience is the Palacio San Martin which is still the formal space of the Foreign Affairs Ministry which is open at times for tours. They are sometimes published in the BA Herald.

http://www.mrecic.gov.ar/portal/psm/home.html