Knossos

Or, rather, the archaeology museum in Heraklion. Most of the art and artifacts worth seeing at the palace of Minos at Knossos are actually in the museum, like this fresco, rather than at the site itself.

The site is still very much worth visiting, but if you are expecting another Ephesus or Olympia, you will be very disappointed. It's much, much older, for one thing; parts of the palace were first built around 1900 BC, and it was destroyed in 1350 BC. So while the size of the site is mind-boggling—archaeologists now believe that the myth of the Labyrinth was based on the palace itself—a lot of what you will see is ditches and stone walls, with some reconstructed elements to give an idea of what the palace looked like a few thousand years ago.