The Hagia Sophia. Construction began in the year 523, and the temple still stands today. The temple was rebuilt several times, its dome collapsed in the year 989 due to an earthquake, and was later rebuilt. Just imagine how many years ago this temple was built, and how fantastic it looks to this day - it's really breathtaking.
If you like temples, take a look at the Monastery of Chora (Chora Museum), the predecessor of the Hagia Sophia. Even though it looks pretty unremarkable from the outside, absolute unreal beauty is waiting for you inside.