The sentence states that "dictatorship, albeit benign," was considered an ideal form of government in the ancient world, even by Plato. This immediately tells us that the blanks should describe types of regimes where power is concentrated, often in a single ruler or a small group, and which can be synonymous with or lead to dictatorships.

Let's examine the options:
- A. Institutional: Refers to established organizations or systems. While regimes are institutional, this word doesn't specifically describe their nature as concentrating power.
- B. Autocratic: A system of government in which one person has absolute power. This is a very strong fit, as a dictatorship is a form of autocracy. Ancient monarchies and other forms of single-person rule were common.
- C. Oligarchic: A form of government in which power is held by a small group of people. While not a single dictator, oligarchies were also common in the ancient world (e.g., Sparta), and they represent a concentration of power that could lead to or be considered a form of tyranny by some ancient thinkers.
- D. Despotic: Relating to a despot; characterized by the exercise of absolute power, often in a cruel or oppressive way. This is very similar to autocratic and directly relates to the idea of a dictator. "Despotic" emphasizes the tyrannical nature often associated with dictatorships.
- E. Anarchist: Characterized by a lack of government or control; this is the opposite of a dictatorship.
- F. Celestial: Relating to the sky or heaven; this is irrelevant to forms of government.

Both "autocratic" and "despotic" strongly align with the concept of a dictatorship and the idea of concentrated power that was prevalent in the ancient world. They are often used interchangeably to describe absolute rule. Plato himself discussed tyranny (a form of despotism) as one of the degenerate forms of government, but he also considered a "philosopher-king" (a benevolent dictator figure) as an ideal.

Given that we need two answers that fit the description of regimes where power is concentrated and which align with the idea of a "dictatorship," "autocratic" and "despotic" are the best choices. They both convey the idea of absolute rule. While "oligarchic" also represents concentrated power, the sentence specifically mentions "a dictatorship, albeit benign," which points more directly to single-person or absolute rule.

Therefore, the two best answers are B. Autocratic and D. Despotic.