Out of them, the only one I can get and want is... Not really any of them because Zygarde and Rayquaza are more my thing, but if I had to choose it was Victini who is only obtainable after certain things.
I can argue that Zygarde exists to restore order, not maintain it, and Xerneas and Yveltal when they appear, pretty much mean their respective versions of the world ending, and if Groudon or Kyogre appear, one or the other causing each other to appear, they fight, and Rayquaza breaks their fighting up, but doesn't maintain anything.
The thing is, is that many of the legendaries don't 'maintain' anything, they just appear when they are needed. Then again, they're all just really damn powerful anyway and do hold a status of being essentially unobtainable.
I can argue that Zygarde exists to restore order, not maintain it, and Xerneas and Yveltal when they appear, pretty much mean their respective versions of the world ending, and if Groudon or Kyogre appear, one or the other causing each other to appear, they fight, and Rayquaza breaks their fighting up, but doesn't maintain anything.
The thing is, is that many of the legendaries don't 'maintain' anything, they just appear when they are needed. Then again, they're all just really damn powerful anyway and do hold a status of being essentially unobtainable.