Every site here is large enough to reshape a day, not fill a gap in one. You don't "swing by" the Acropolis on the way to lunch. You don't see Meteora in forty-five minutes. Mycenae's Lion Gate and beehive tombs take a full morning to walk properly, and Akrotiri's covered excavation, despite being compact compared to the others, demands slow attention to streets and frescoes most visitors rush past in twenty minutes flat. Scale is the first filter. If a ruin can be seen in under an hour and understood in less, it didn't make this list.
The second shared trait is that each site rewards prior knowledge disproportionately. A tourist who shows up to Delphi without knowing what an oracle was, or why Apollo mattered more here than almost anywhere else in the Greek world, will see a hillside of broken columns and nothing more. A tourist who understands the Pythia's role, the Sacred Way, and why city-states funded elaborate treasuries here to outdo each other walks away with something closer to the actual significance of the place. These sites punish laziness and reward preparation more severely than almost anywhere else in the country.
Where they differ matters just as much. The Acropolis is civic and political — a fortified hill turned into a monument to Athenian democracy and military confidence after the Persian Wars. Delphi is religious and pan-Hellenic, neutral ground where rival city-states competed through architecture rather than armies. Meteora is monastic and geological, a Byzantine answer to isolation built into sandstone pillars that look improbable even standing beneath them. Mycenae is Bronze Age and martial, predating classical Greece by nearly a thousand years, the world of Agamemnon and fortified palaces rather than temples and democracy. Akrotiri is domestic and commercial, a Minoan-era trading town buried by the Thera eruption, offering streets, drainage systems, and multi-story buildings instead of temples at all.
Together they span roughly two thousand years of history and five entirely different reasons humans built monumentally in this part of the world — worship, war, governance, trade, and retreat. That range is the actual argument for including all five rather than picking one or two "representative" ruins and calling it done.