For us, and many, the Valle dei Templi - although don't get confused, the temples are on a ridge of land and not in a valley. The collection of temples on the ridge are outside of the town and a sight you will not forget.
There are eight temples (and various other remains) built between about 510 BC and 430 BC: the Temple of Hera, the Temple of Concordia, the Temple of Heracles, the Temple of Olympian Zeus, the Temple of Castor and Pollux, the Temple of Hephaestos, the Temple of Demeter, and the Temple of Asclepius (the God of Medicine). The temples of Hera and Concordia are the two most complete temples.
The Temple of Heracles is the oldest. There are only nine columns still standing but you will find this one most impressive as they are tall columns and you easily get a sense of the size of the original building.
The four columns of the Temple of Castor and Pollux are famous as they are some of the photographed works of the site.
As you walk through the remains you will feel that lump in your throat, you will get a sense of how little time we represent and you will be in awe of Agrigento. This is the mother-load of ancient buildings. As I don't think there is another single location that provides the number of arrow of buildings and ruins.
We have had our experiences here. Once we parked and then walked up to the area of the temples as Glenn wanted evening shots. That was the easy part. But we did get lost trying to get down and find the parking lot. All I can say it is very very dark.