Overrated: Montesquieu. The separation of powers he extolled in practice amounted to the rights of minor aristocrats to rule petty fiefdoms and entrenched powers to run rampant and unchecked. In general (obviously not always, don't @ me with counterexamples), common people have far more to fear from local despots rather than supposed all-powerful tyrants who, for all their flaws, have to at least govern taking everybody into account. It's no wonder that he found his origins in the Parlements of the ancien regime, the aristocratic institutions who consistently blew up all royal attempts to get the house in order by claiming this itself was tyranny, an obstinacy that would completely blow up in their faces in the Revolution. Moreover, his influence on states all over the world has been largely counterproductive: constitutions jammed fill with checks and balances and heavily separated powers are no less unlikely to be led by tyrants than states without; the idea that all branches of government jealously guard their own privileges against the other leading to a neat equilibrium is one that is not seen in reality.
Yeah, I'm all in favor of the primacy of local governance and avoiding concentrations of power in one individual or institution(s) as much as possible, but in practice, Montesquieu's philosophy has led to redundant bureaucracies, completely unnecessary gridlock, and glorified palace intrigue as every schemer and operator who wants to be in or close to power competes to get as much as possible.
To use one of my favorite examples: the pre-MBS Saudi Arabia, where the interests of tens of thousands (!!!) of inbred royals, a rigid clerical establishment, some important merchant and tribal leaders, and many comically bloated and overlapping state ministries/fiefdoms were mediated by the King and his brothers (the current monarch being the primary mediator/enforcer, FWIW). Real paragon of republican virtue there! /s
