Many social and political thinkers begin with this question without recognising that their very starting point is already ideologically loaded. In fact, the question is inherently collectivist and authoritarian. My own view is that any dictator, no matter how benevolent, would be unjust, and a "just dictator" is an oxymoron.
A wholly just person would never end up in a position of absolute political power, and if such a person would somehow magically be placed in such a position, he or she would resign immediately. Of course, no human being is completely just, so for any human, the temptation would be too strong to try to use such a position of power to "do good", as well as to bolster and reinforce their own power. Power corrupts and absolute power corrupts absolutely. And, I claim, even just asking yourself the question "What would a benevolent dictator do?" can have a corruptible influence on your thinking. Especially, when you imagine yourself as the benevolent dictator.
The basic idea of so-called "social choice" theory is to add together the preferences, interests, or welfares of separate individuals to make up a "social welfare", and to reach a collective decision. This inevitably involves some choosing for all.
Another similarly loaded question that is often being asked is "How should we divide society's resources?". What is the "we" here? Usually, it is either implicitly or explicitly taken to be the government of the state (or perhaps an imagined "world government"). It doesn't help to rephrase the question as "How should society's resources be divided?", because this rephrasing only masks the still present appeal to some entity that supposedly has the right and ability to command all resources in society. Furthermore, it must always be remembered that governments don't have any resources of their own. Whatever the government spends has first to be taken from someone else.
By starting with such loaded questions the more fundamental question of the very legitimacy of political authority is evaded and ignored. But it is this latter question that should be the first one to ask in political philosophy.
A great way to start exploring that question is by reading Michael Huemer's excellent book The Problem of Political Authority: An Examination of the Right to Rule and the Duty to Obey. Get a quick overview of the book by reading the very helpful analytical table of contents.