I've never played Sidar, Kurio, Balseraphs, Clan, Hippus or Mercurians, so I don't really have enough experience to back up my thoughts, but I do have some preferences.
I do not like Doviello at all, I really hate early war and in the one game I played as Doviello I screwed that part up and ended up falling apart with no money and no research. My fault, I know, but I am just not good at that playstyle and don't enjoy it. I can see Clan being less painful, though, plus orcs are cool, and anyway I haven't tried them.
Hippus are just horribly boring to me, I utterly do not care about their theme at all and I find that more important to my enjoyment of the game than the civ's actual effectiveness. Hippus are just, bleh.
Balseraphs are evil clowns. They are just way too freakin creepy for me to play. I've tried quite a few times but I never really like the games I start and the whole civ is just not appealing.
Kuriotates also seems pretty bad to me, but I'm scared of their mechanics and I've never actually tried to play as them. Their units actually seem pretty cool, I just am scared of the city cap. I'd like to try them out some time but I haven't really been motivated enough yet to figure out how to run them effectively.