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The Federation
Standing as the shining bastion of democracy and freedom, the Federation is the oldest superpower. Rising from the ruins of Earth, the Federation values equality, liberty, and self-determination. Driven by entrepreneurship, entire generations of Federal citizens remain loyal to the benevolent corporations that have brought them such prosperity - wait, hold on. Someone slipped some propaganda into my script. I do apologise.
The Federation is a corrupt dystopian nightmare, by far the worst of the three powers. The average citizen is a wage slave, their only freedom being what branded products they consume. Corporations have massive influence everywhere, but it is the Federation that is truly ruled by them, its society and politics exploited for profit. Politicians who don't bow before the corporates are assassinated by them, all the way up to the President. Political deadlock can arise as corporates and their chosen politicians fight to decide which corporate interest is most deserving of the latest round of government support, while the citizens are left to rot on the streets should their life no longer be profitable enough.
The Empire
Born from the rebellion against the corrupt Federation, the Empire of Achenar is a place of honour. Ruled by the benevolent Emperor, it is considered shameful to mistreat others, and the political system requires Senators to represent the interests of their people, lest they lose their power. The Empire looks with disdain at the lesser powers that allow their citizens to live in squalor, with Imperial Slavery providing a safety net for those at the bottom to work their way back up- oh. It's happened again. I am SO sorry.
The Empire is a corrupt dystopian nightmare, by far the worst of the three powers. The honor system chains the lowest in society while allowing freedom to those powerful enough to be dishonest. Senators are above the law, save for the Emperor's will, and the most influential of them gain their support by exploiting others. Freedom of expression is non-existant, if you have the wrong opinion you can lose your job, your home, your family, your life. To oppose the Emperor would be dishonourable, after all. Yet even the Emperor cannot act freely, for the stench of corruption that infected the Federation has its claws here too.
The Alliance
Breaking free from both the Federation and Empire, stands the Alliance of Independent Systems. An Alliance of equals, it aims for mutual prosperity of all its members through cooperation and freedom. Each system has the right to self-determination, while still benefiting from the shared values of the Alliance. A smarter, fairer democracy, with free and happy citizens in control of their own lives. Nobody slipped this into my script, by the way. I wrote this. The Alliance supporters never got around to sabotaging my script, I think they're still stuck in another meeting, so I just did it for them.
The Alliance is a corrupt dystopian nightmare, by far the worst of the three powers, if you count it as a power. Bureaucratic and slow, it frequently takes the enlightened middle route of doing absolutely nothing. Democratic deadlock is the norm, as getting enough systems to agree to anything is near-impossible. And despite what they claim in their leaflets, the Alliance is not immune to the flaws of the Federation and Empire. Corporations hold increasing sway, with the Prime Minister's term lasting unusually long thanks to backing from the colossal Sirius Corporation, which enjoys various benefits in return. Authoritarian governments and outright dictatorships can hold Allied membership, the wider Alliance unable to actually enforce its shining ideals on its own members.
Independents
"Wow, all the powers suck, i'm going to go independent" - yeah, sorry, these guys suck too. Elite is a dystopia.
The Club
There is no secret Club that manipulates all of humanity in their own interests, you're just being paranoid.
Miscellaneous
For the amateurs who can only manage one or two galactic conspiracies, not even enough to warrant their own section.