Solo, National Archaeological Museum works.
For anyone who cares about antiquity this is the deep, quiet, world-class payoff that the crowded Acropolis can't give you — room after room of things you've seen in textbooks, often nearly empty. The catch: it's a classic old-school museum out in a grittier neighbourhood north of the centre, it's big enough to exhaust you, and casual visitors can find it relentless.