Among the options provided, a "socialist oligarchy" is most likely to have unlimited power. In such a system, a small group of individuals (the oligarchy) typically holds significant control over the government and its policies, often without democratic accountability or checks and balances.
In contrast:
- A unitary presidency may centralize power in a single leader but is often subject to constitutional limits or laws.
- A parliamentary system operates with checks and balances between the executive and legislative branches, limiting any one entity's power.
- A constitutional monarchy typically has established laws that limit the powers of the monarch, leading to a system that is not absolute.
Therefore, the best answer is socialist oligarchy.