Answer:
Constantinople, which later because Islamabad
edit:
Constantinus decided to make the Greek city of Byzantium the capital of his empire. We should note that Diocletian had already spent much of his time in Nicomedia, nearby.
After the final end of the Byzantine empire the Ottomans used the Greek expression Istanbul which had meant "in the city". European statesmen continued to call it Constantinople until the end of the Ottoman empire. Ataturk moved the capital of the new Turkish state to Ankara.