The Kariye Camii in Istanbul, Turkey; also known as the Church of the Holy Saviour in Chora is the most remote sight I visited in the city. It was a recommendation of my hosts and it really was worth the detour to the northwest end of Fatih. It was once a Byzantine Christian church that was converted into a mosque and is now serving as a museum.
The church is quite small and dates back to the 5th century CE. It is impressive because of the large mosaics added added after the year 1315 and covered during Ottoman time as an effect of the aniconism in Islam. From the year 1984 on the mosaics have been uncovered by a restauration program founded by American institutes.
Church of the Holy Saviour in Chora