The New World Order is actually the one conspiracy theory that may have some merit to it. The thing about a one world government though is, it is not immediately apparent in and of itself why it is a bad thing. Or at least why those that aspire to form a one world government are doing so for nefarious ends.
The reason I say it may have some merit is I have heard/read some things over the years that give it some credence. For instance, Jon Ronson years ago interviewed one of the founders of Bilderberg, and asked him if it was true that one of the aims of Bilderberg was to create a new world order. And the strange thing was he basically admitted it (lol). However, it was said as if like, what is wrong with that? A world without countless wars, etc.
So yeah, there may be some merit to it, but at the same time, the reason why someone might want a one world government isn't necessarily morally questionable either.