One person with superman-like powers could rule over a huge region, maybe the world. Let's first discuss ruling their own nation though, that scale is easier.
Our metahuman can't police the nation, that is impossible even with those kinds of powers, but what the metahuman can do is defeat conventional military forces or concentrations of power. Our metahuman can deliver a military victory essentially, but as we often see, there is more to winning a war than that. In order to maintain a new regime, there has to be a large infrastructure of politicians, police, military, bureaucrats and so on. Even if the metahuman can completely topple the current government, he can't ensure his vision comes to pass without the above help of many thousands of people. The easiest scenario for the metahuman is to keep the current infrastructure in place, but merely take it over, to force the politicians, bureaucrats, police, and others to do his bidding within the existing hierarchy but with him as the new top.
Can the metahuman accomplish that? Yes, probably so. He just needs to make it clear to the current leaders that they obey or die. There may well be a lot of resistance, but it beats starting from scratch. For an ideologically motivated metahuman, it might be good to install ideological allies in positions of power to help enforce the rules of the new regime.
Assuming the metahuman can successfully do this, then we get to what they do with their leadership position.
Some poor kid in the middle of centroamérica whose parents were shot by narcos
So, if the kid comes from an area where the narcos just obviously rule, everybody knows who they are and where they are, then the kid can make short work of them. The narcos might be more subtle that that though, and that is where the kid can run into problems. What if the narcos kidnap the kid's surviving family and force compliance or they will kill or torture them? What if the narcos have so many authorities bribed that the kid can't get anybody to honestly enforce his rule? This is a tricky scenario that might require more knowledge and thought than our poor orphan has. Maybe the best way for him to deal with the cartels is by finding some honest members of law enforcement, maybe even in some other nation where the cartels have less influence, and getting as much information as he can from them and then using that information to take down the cartels. Imagine the kid showing up at CIA headquarters and asking them to research cartels for him.
Far-right incel who is extremely online and has no friends
Well, I suppose in an imaginary world where superpowers exist, our theoretical "far-right incel" can too.
This depends a lot of what "far-right" even means. Incel suggests that it's an anti-feminist, maybe MGTOW, sort of guy who (matching the stereotype) dislikes women as well as feminism and resents not being desired. That stuff isn't necessarily even far right really, well, aside from opposing feminism. This sort of incel stereotype implies that the guy isn't successful in life, and the OP says he has no friends, and presumably no female companionship in his life.
Well, that will radically change when our incel gains superpowers. He will be an instant celebrity and even if he's really unattractive both physically and socially, he is going to have as much sex as he wants. He doesn't have to rape anybody, because I'm sure that there are more 8+ girls who would jump at the chance to be in God's harem than he can handle. He's also going to get a lot of friends too, or at least people pretending to be his friends to get something from him or to be spared his wrath.
That could significantly change the psychological profile of our incel if not his theoretical political views. Would this guy be content just to be a hero and get all of the fame, glory, and girls? Or, would he risk becoming hated by many to enforce a very unpopular viewpoint on the world? At some point, I assume, he would want to try to influence the world and the affects of that would depend on his political agenda. Just to keep in mind though, someone with superman powers isn't going to be an incel for long.
Far-left college student whose buddies are all black-bloc types
Well, this may be the easiest to predict the outcome of and also the most likely negative outcome. We know what happens when communists take over a country, we've seen it played out far too many times. One of the big questions here would be if our super-antifa is going to have a chaotic change or an orderly one. There are already huge numbers of left wing extremists in most powerful institutions in Western nations. There is already an infrastructure in place for our commie dictator to rule over and countless foot soldiers to enforce his rule. Does the metahuman just take over the government and purge the conservatives and moderates? If that is the case, then we just go straight into a communist regime that we have all seen before, maybe with more dysfunction though because the dictator would be so extreme, ideological, and detached from reality. Global warming is destroying the planet - ban all fossil fuels and
do it today! All businesses must be at least 51% female. No white police should ever arrest a "person of color." Not even Mao's China or Stalin's Soviet Union had to deal with such mad policies as those or the chaos they will create. But capitalism is global, traditional values are global, industrialization is global, so are evil white males. There is no way that our super-antifa is going to be content to rule only one nation and as long as there are non-compliant polities out there, he will blame them for his nation's failures. This metahuman will want to, as one extreme example, globally ban fossil fuels immediately. That could result in billions of deaths. Could he be reasoned with? I don't know, maybe.
If this metahuman wants to have a chaotic change, then things go downhill even faster. Police are banned. Power plants and car factories are destroyed. Conservative and moderate politicians are killed. Dissenting media pundits are killed. An anarcho-socialist might expect that a utopian society will immerge when all of the police, businesses, and capitalists are gone - but he will be disappointed as the true effects are apocalyptic.
Maybe I'm assuming that this person is too extreme, but if the kind of leftist who beats up Andy Ngo for being a "white supremacist" got superpowers, then no abuse of power would surprise me.
An actual politician of reasonable social life and normal upbringing in some Western European country, or a similarly westernized nation like Israel or Republic of Korea, preferably of a party that isn't getting its way
This might be one of the more moderate possibilities. It depends on the agenda of the politician. In the USA, a political party not getting its way would be Libertarians. How would a superpowered libertarian enforce his will on the nation? It's hard to say, though too much use of force would be hypocritical. A politician might be able to get a lot of power just by using the superpowers for good and using his platform to spread his political beliefs.
Assuming the metahuman actually takes control of the nation, this scenario is the one where we are most likely to see a successful taking over of existing infrastructure. The government could remain in place, along with military, police, bureaucrats, military, and so on. If this person is a politician already, their agenda might be moderate enough that it can be inacted without too much oppression or bloodshed, depending on what his politics are. Though far too many supposedly moderate left wing politicians might be dangerously close to the person above.
A farmer who doesn't like rice prices
I would need more information to go on. A metahuman won't need to personally worry about rice prices anymore, but someone who cares about such issues could have countless ways to go about remedying what they perceive as a problem.
