Among the options provided, a socialist oligarchy could be considered as having unlimited power, as it typically involves a small group of people (the oligarchy) controlling the state and its resources, often without the checks and balances that are present in other systems.
In contrast, a unitary presidency and a parliamentary system usually have some degree of checks and balances, and while they may have significant power, they are not typically characterized by an absence of limits on that power.
Therefore, the answer is socialist oligarchy.