If you play around with Roleplay a good amount, I can second the Commonwealth.
Other than that? Anywhere except liberty I can say really, it all depends on what approach you want for any character you would play. How you want to play them, etc.
Perhaps the Unione Corse as well including Gallia?
Again, I'd go Commonwealth (more advertising from members hue hue). I'm currently the member of two unofficial Rp factions (guess the other) and out of anything I've ever been in I'd say CW provides a certain amount of Rp freedom which really suits certain people. For instance your character doesn't need to take a certain ethical stance; they could be outgoingly evil or beligerantly good, it doesn't matter. Crazy, slightly different Rp strains are encouraged and individual character stories often end up being tied in to the faction as a whole. For example: I remember a certain time when we were loading up our whales with Niobium in Tau IMG (23), and a few 101's pirated us (probably owing to the fact that IMG was filling us, OC's usually leave us alone). What immediately followed was five minutes of intense Rp negotiation which ended with the CW guys not only getting away with short-changing the pirates, the pirates leaving, Robert (Ed's character) complimenting the Maltese on their manners, Achille (my character) announcing a realisation (in system chat, all InRp), that the pirates may have forgotten a certain financial transaction concerning his person, and then the entire convoy regrouped and ran through Omicron Alpha as fast as our little cruise engines could carry us.
If talking your way out of everything seems more amusing than pews, then you might want to take Ed's/Moveit56's/Achille's (my) offer up. Again, shameless advertising.
ohh..adn yeah [C] Council is dropping activity....You might be their Saviour..Since we (GRN|) have some Plans that might increase the activity of both Factions