I didn't vote for DS just because I've never played with them, but I suspect they be one I would have voted for if I'd had a game or two with them. Other stuff I didn't vote for and why I don't pick them:
ZM - Too much pressure. You get only 30 minutes warning, they hit 3 systems, and have 120 million hp each. You will almost always lose the first three targets. If I played with no fog so I could see all the worlds, it wouldn't be as annoying. I might risk them up to 8/8, but honestly, they aren't a very fun challenge. It is like playing against a lottery where all the results are bad.
NRC - Eh, kinda annoying, but also kinda ignorable. Not really interetsing enough. I think my biggest issue is the warheads are random. EMP are the big threat, Lighting not at all. I'd rather if 2/3rd of all spawned only Lightning (maybe faster than currently) and the other 1/3rd only spawned EMP. At least then I know whether it will be a minor in inconvenience or downright nasty. And they suck on Adv. Factory worlds.
Wardens - Seem broken, as in did nothing my last game. Seem like a poor very of the Hybrid, so I'd rather take Hybrids.
Enclaves - Ever-present harassment, but not really threatening. Again, Hybrids are more interesting.
Devourer - Too cheesable for the player, especially with SF and threat fleets now. I can avoid him, AI gets munched. Plus, I think it is cheap it has a ranged attack and isn't melee or tractor-based.
SCL - Cheesable in full conquest games, very annoying otherwise. I'd rather just turn on AIP/60 minutes. Killing the AI CS to make them friendly but not attackable by the AI is...silly.