The Temple of Neptune — formally the Second Temple of Hera, but almost nobody calls it that — is one of the best-preserved Doric temples anywhere in the world. Built around 460 BC during the height of the Greek classical period, it has 36 columns arranged in the standard peripteral plan, and the entablature above them is nearly complete. You can walk between the columns into the interior, where two rows of smaller superimposed columns once supported the roof; the stumps of those inner columns still stand.
The proportions here are confident in a way the older Basilica beside it is not. The Archaic heaviness is gone — the columns are taller, the capitals less exaggerated, the whole thing more upright. Standing at the end of the long axis and looking through the colonnade, you get a sense of why the Greeks considered this the standard against which all temples should be measured. It is, bluntly, magnificent.