Secretary-Generals of the U.N.
While individual U.N. international agencies accomplished many positive results (i.e. elimination of smallpox, international telecommunications standards, etc.), the U.N. was always hampered by having no power of enforcement. Member nations could (and frequently did) ignore U.N. resolutions that were inconvenient.
By the time the Nexus gave teeth to the United Nations, the U.N. was so mired in bureaucracy and internal politics as to be almost completely ineffective.
In the crisis of 2042 CE, the UN's authority was transferred
to the Terran Council.