I believe that the Elgin Marbles should be returned to Athens.
When the marbles were taken, it was not obvious that such an action was legal. The wording of the documents is unclear, and the marbles were not initially accepted by England. However, I believe the action was slightly justified because the Acropolis was in a terrible state, with many statues having been recently destroyed. Taking them away was a way of protecting them.
However, I believe that this justification no longer applies. In the present, Greece is a developed country that, while not rich, has enough resources to take care of its heritage. It is unlikely that the marbles suffer any danger in Greece, and therefore, the marbles can now be housed in the place where they came from.