Well, if I had to rank them, I'd put them as:
1. Frano-Ibera: I've always been huge into culture and tech, so the second I heard about them, they seemed like my perfect fit.
2. Polystralia: I'm also huge into the economic side of empire-building (Trade Routes OP

), and the Hutama interview was hilarious
3. PAC: WONDER SPAM!!! In all seriousness, it appears that worker speed is much more important in BE than CiV.
4. Slavic Federation: Longer duration on orbital units is nice, but the real deal here is that early free tech, lots of interesting opportunities with that.
5. PAU: I like a nice n tall civilization, and these guys can make that happen (though I think that others like FI & 'Stralia are better...). They also have the flexibility to make that "wide" empire a littler "taller" if you manage your health decently.
6. Kavithan Protectorate: Nice bonus (esp after their buff, much needed) if I'm even in the mood for a bunch of quick land-grabbing. Arguably going culture-focused with a different sponsor (or that prosperity virtue) will mimic the result, but maybe that's their strength: not HAVING to take those measures as their borders will grow quickly anyways.
7. ARC: While I've seen the power of espianage (and am impressed by how they spiced it up compared to CiV) I'm drawn toward the others as they more defined playstyles. Like I mentioned with KP, maybe that's the point, is that they're so flexible, you do what you want.
8. Brazilia: I can echo the "bland power" comment. Useless power? No. Good for early alien killing? Yes. Good for late-game wars? Hey, 10% extra is nice on bigger combat numbers. You can probably guess by me ranking FI & 'Stralia up top, that I'm not a huge warfare guy (I prefer to win economically/technologically) and this guy's bonus can be aquired (arguably) easily enough by anyone else in a manner of speaking.