More like the UN than any of these. But of the lot, it’s the US. Bicameral legislature without something like Britain’s House of Lords for nobility or aristocratic families. Rome was far too aristocratic, oligarchical and discriminatory in voting to match. And Athens was a direct democracy, which is the antithesis of the Republic’s bureaucracy.
City-State of Athens
Roman Republic
Civil War-Era America
World War II-Era Britain