Past ownership aside (since that's just a Pandora's box of claims that would individually cause ruinous wars and together probably just end the world, more or less), 'should' absent the terribly subjective 'moral' aspect probably implies what's best for the inhabitants of both the city, the surrounding area, and the nations around it, no?
In which case, Turkey was probably the best choice, seeing as they didn't get involved in WWII and were at a level of power simultaneously sufficient for deterring invasion but inadequate for leading offensive revanchist wars against its neighbors, unlike the rest of the list (aside from the 'free city' option). Not that all of the other candidates were necessarily willing to get embroiled in conflict(s) but the circumstances of their involvement would likely remain unchanged absent any other POD and could potentially even exacerbate the interwar situation even more (Bulgaria-Greece, for example). And, while a free city would be the most democratic and fairest option in an ideal world, the reality would probably be a rather ugly series of conflicts for the city, seeing the fate of most other free cities that were highly desirable by one or more neighbors. Under Turkish governance, Istanbul avoided the ravages of the worst war in history, something none of the other candidates on the list can boast.