Athens and the Acropolis
Yes, the Parthenon is amazing in many ways and I am glad that I did see it ... once.
But once you get there you realize that the place is not very well maintained, the most recent restoration effort goes unfinished after 30+ years, and the grounds are inhabited by dozens of feral dogs who have discovered that wherever the tourists roam, food waste can't be far behind.
I'd contend that it is worth a trip to take it off your bucket list but, if planning a vacation to Greece, I'd spend much more time on the islands and far less time in the city.