Acropolis
The "jewel of the world". You have to at least have seen it once in your lifetime, a bit like the Eiffel tower. It's worth it. Best go first thing in the morning or at the end of the day, otherwise it's too hot. And also book tickets online with a time slot, so you don't have to wait in queue!
Best access through the southern slope. You have to walk up anyway, might as well see the Dionysos & Odeon theaters along the way.