Oligarchy, aristocracy, plutocracy are comparable when they mean government by, or a state governed by, the few.
The terms are often applied to governments or states that are ostensibly monarchies or republics but are, in the opinion of the user, actually governed by a clique.
Oligarchy is the most inclusive term referable to a government or state where the power is openly or virtually in the hands of a few men.
Aristocracy basically and historically suggests the rule of the best citizens, but it retains this implication chiefly when it is used in distinction from oligarchy and the latter connotes power seized or held for selfish or corrupt reasons.
Its more common implication is power vested in a privileged class, especially in a nobility that is regarded as superior by birth and breeding and that by owning or controlling much of the land exercises direct control over a large portion of the population; in this sense Great Britain was until recent generations an aristocracy.
Plutocracy, unlike the other terms, is usually derogatory; as a rule it implies concentration of power in the hands of the wealthy and, in consequence, a withholding of power from those to whom it properly belongs, either the people or their representatives.