The acropolis, which is viewable from every angle of the Syntagma / Plaka and most of Athens is the prime attraction in Athens and is the one historical site you can't miss.

Acropolis houses the PARTHENON, the symbol of Greek rule for the last couple of thousand years.
It can be rewached by a short walk from Syntagma / Plaka ( about 15 minutes) or by taking the metro to Acropolis and then a 5 minutes trot.

You can take a tour or wander up there yourself but during the summer, whatever you do, go early or late in the day. It can get very hot up there and gasping for breath can take way from your ability to marvel at the greatest of all archaeological sites.