In 1930 Antonis Benakis – a politician's son born in Alexandria, Egypt, in the late 19th century – endowed what is perhaps the finest museum in Greece. Its three floors showcase impeccable treasures from the Bronze Age up to WWII. Especially gorgeous are the Byzantine icons and the extensive collection of Greek regional costumes, as well as complete sitting rooms from Macedonian mansions, intricately carved and painted. Benakis had such a good eye that even the agricultural tools are beautiful.
The variety of displays here, including Classical Greek masterpieces, means that it's great for a group with varied interests. Late opening hours are another plus.
Note also that there are two other major branches, 138 Pireos St for contemporary art, and the Museum of Islamic Art in Keramikos, as well as several smaller museums. A €25 pass to all is valid for three months.