That is a complex question that deserves to be answered. Personally, I believe that the Eastern Orthodox, Oriental Orthodox, and Assyrian Orthodox Churches have equal right to consider themselves Orthodox. The schism of Ephesus did not occur because the Assyrian Christians were Nestorian, that was an unfortunate misunderstanding. Furthermore, neither did the Chalcedonian schism occur due to the Oriental Orthodox Christians being Monophysite, that was another unfortunate misunderstanding.
Steps have been taken, and are still ongoing, for all Orthodox Christians to be re-united. This is not part of the ecumenical movement and should not be confused as such. This is fellow Orthodox Christians making the conscious decision to not repeat the mistakes of the past. Even when apart, we are united by the doctrinal and liturgical tradition of the faith.