U.S. Ambassador to the United Nations
The U.S. Ambassador to the United Nations is Chief of the U.S. Mission to the United Nations. This Ambassador, depending on her or his relationship to the President often has courtesy Cabinet rank and thus senior to other Ambassadors. This occasionally leads to conflict in status with the U.S. Secretary of State, but the Secretary of State almost always will win a dispute.
The incumbent is Susan Rice.
Other U.S. diplomats may represent the U.S. to be other U.S. diplomats with ambassadorial rank at U.N. organizations, but they do not have Cabinet status.