One world government
In theory, there's nothing wrong with planetary federalism. Most Heinlein books assumed something like that.
Problem is, our federalism was invented by people who believed in individual liberty as a primary. Most modern "one world government" people and groups are collectivists and Marxists. Look at the people on the top in the E.U., and the "constitution" they tried to pass.
Our Constitution was a very short document declaring the few things that citizens couldn't do, and the few things that government could. If someone proposed a world government on that basis, I'd listen. Not holding my breath.