Hidden inside the Theodosian Walls, Kariye Camii contains the finest surviving Byzantine mosaics and frescoes in Istanbul — arguably in the world. They date from around 1315–1321, commissioned by the statesman Theodore Metochites, and depict scenes from the lives of Christ and the Virgin with astonishing detail and colour. Most visitors skip this for Hagia Sophia; that's their loss.
Chora Church (Kariye)
Finest surviving Byzantine mosaics in Istanbul, far from the tourist crowds