Arbiter

An arbiter is a fair and impartial determiner of fact.

Who appointed you as the arbiter of truth?

Google has become the de facto gatekeeper of the web, an arbiter of what is useful and what should get visibility.

In order to protect individual liberty there must be an arbiter between the governing powers and the governed.

Television and film, not Vogue and similar magazines, are the arbiters of fashion.

The grapholect of Standard English is not the exclusive system that arbiters of cultural purity wish to 'correct' us into believing

The dreadnought is the ultimate arbiter of space warfare; millions of tons of metal, ceramic, and polymer dedicated to the projection of firepower against an enemy vessel of like ability. No sane commander would face a dreadnought with anything less than another dreadnought.

Worse, since there was no institution to arbiter disagreements between Parliament and the government, whenever Parliament voted against the government on the smallest issues, coalitions fragmented, and governments had to be recomposed.