The Pyramids at Giza. Not only because of their construction (which still astounds us, as does the construction of Stonehenge) current theories indicate they were also used as observatories as well as burial chambers. Included in that array of construction is the Sphynx. Not only does the area now give us some idea of Egyptian astronomy, the area itself was critical in understanding the Egyptian language with the discovery of the Rosetta Stone in 1799.
Jerusalem is an excellent answer. I immediately thought of the pyramids, which are among the most astounding of all human achievements. It seems almost incredible that such structures could be built given the technology that existed at the time.
I would say the city of Jerusalem. It has high religious significance in many religions, and there are several monuments of religious and historical importance to different cultures and religious groups. That is why everyone still is fighting over it!