All the Empire needs is a couple of Spartacuses to bring it down. Oh, and a few crazy emperors, but I think that's covered already.
The Federation... Well, I hate the idea of corporate overlords telling me what I should wear, think, breath, watch, like etc. A couple of independent journalists should bring it down easily enough.
The Alliance will never grow to become a superpower, because they lack focus as a result of their decentralised government. They provide each other safety, however, by combining their resources to purchase and wield heavy weaponry. Relatively nice place to live, I'd guess, if you can eradicate the inevitable corruption.
The Independents... Each one a different, unique experience... Totalitarian governments next to crazy religious cults and various concepts of utopia... That should provide some fun for the adventurous soul!