Mh, the Hegemony has the coolest planet with Eventide. They are also the most powerful and thus most useful for the player.
I'd like TT because of the AIs (AIs get me excited), but you don't actually see much of them. High tech ships are nice, but not a must for me anymore.
The Church and the Diktat are not yet fleshed out enough to get a good impression. The indies are not unified enough, but I might yet become a fan of one of their members. Pirates might grow on me when I have played as one long enough.
Then there are some other factions yet to come, e.g. the Persian League might become really interesting.
For now the Hegs win the race, but that might change soon