I'm going to leave a handful of thoughts here on some of the presented suspects. Some of them have probably been already stated by others, but well, that's okay.
First off, Darkrai, who is in my opinion the most dangerous of the possible ubers.
The new sleep mechanic, combined with Breloom and Darkrai, along with several other less common sleepers (hypnosis politoed, spore morobareru, spore parasect, as well as a handful of sleep powder using grasses), basically necessitates running a sleep absorber (a resttalker, an insomniac, a vital spirit user, heal bell, or aromatherapy).
One of the main issues with this is that very, very few sleep absorbers can safely switch in to both breloom and darkrai, considering that between them they have nearly unresisted STABs (only heracross, toxicroak, and croagunk resist grass/fighting/dark). Even without this issue, however, no sleep absorber can even somewhat reliably counter Darkrai, factoring in bad dreams, a couple of layers of entry hazards (trivial to get, by the way, with Deo-s, nattorei, or sturdy+custap leads), and either two dark pulses or two focus blasts. The very, very few pokemon that might be able to survive are generally incapable of doing enough damage back to Darkrai to effectively counter it. Scarf Sleep Talk Heracross is a notable exception to this, but for the most part is pretty predictable, easily walled, and with team preview, trivially beaten with double switches (I bring out darkrai, then go to skarmory, for instance, just laying down more hazards to make the darkrai sweep easier later).
Because of this, Darkrai clearly needs both a sleep absorber and a separate pokemon responsible for countering Darkrai. On the switch to your separate counter, Darkrai can easily set up a Nasty Plot or a Substitute (or simply attack), all devastating options. Considering that many of the bulkiest pokemon in the game are weak to either dark (most bulky psychics or ghosts) or fighting (Blissey, Tyranitar, most steels), even without abusing the unreliability of sleep talk or the extra damage of Bad Dreams, Darkrai can often muscle through his supposed counters. Therefore Darkrai's counters (after the sleep have been absorbed by someone else) are mostly left to a handful of priority users and scarfers. Fighting priority is really the only one that scares Darkrai with his reasonable bulk, and either substitute or chople berry lets him break through these opponents. With Wobbuffet, or simply by wearing down the scarfer through repeated attacks on switches and with entry hazards, Darkrai can easily break down most revenge killers with relatively small support.
Because of all of this, I am quite confident that Darkrai should be Uber.
Next, Skymin.
Skymin is much more manageable than Darkrai. A specially defensive Zapdos (with roar, to handle subseed) beats just about everything besides Specs super effective hidden power (usually ice). Specially defensive skarmory beats everything besides hidden power fire. Nevertheless, Skymin will likely become uber. Despite having a handful of viable counters, Skymin outspeeds almost the entire metagame (with a scarf, Skymin outruns even adamant Dory in the sand). This makes it nearly impossible to revenge kill, especially considering Skymin's resists to both water and fighting, both popular priority types with Dory's prevalence. Flinch hax and special defense drops means that only quite specialized counters can expect to win reliably.
Because of this, I suspect that Skymin will be uber, albeit with not quite as much confidence.
Offensive/Lead Deoxys Forms (Deo-A, Deo-S, Deo):
As leads, they excel, although with magic mirror from espeon, fast sleeps, and other competing leads like sturdy+custap forretress, they frankly aren't overpowering there. Offensively, Deo-A is the one to talk about. It does tremendous amounts of damage to almost anything, and although it has a lot of checks, it packs very, very few counters (basically only specially defensive spiritomb with shadow sneak and pursuit). Although priority tends to hurt it a lot, extremespeed helps negate the weakness. Thankfully, a lot of its power comes from superpower and psycho boost, both of which lower its offenses. As such, Deo-A tends to be at least feasible to play around.
Nevertheless, because of the incredible power, coverage, and speed they offer, I suspect at least Deo-A will be uber, with perhaps even more confidence than with Skymin (at least skymin does have counters more viable than spiritomb. . .). Deo-S and Deo would need to be seen in a meta without Deo-A to evaluate their offensive ability accurately.
Others:
Dory: tends to be easily countered/checked by a lot of priority users and a lot of common walls, as well as weather changers. Excellent, but far from overpowering. I'd be interested in seeing if, say, chople berry or offensive spinner variations might be more broken than the more standard LO/Balloon sets, but frankly very few people run either of those.
Politoed/Ninetales: To be honest, I'm not seeing too much broken about them, despite the nice support they offer. I'm inclined to say quite good, but not broken, but have little evidence to support that.
Manaphy: Based on theorymon I would think quite possibly broken, esp. with politoed. To be frank I haven't seen enough of them abused to make a good opinion, though.
Inconsistent Abusers: They sound really, really annoying, but I haven't seen enough to make a good opinion.
Lati@s: There are a lot of really powerful special dragons, and frankly with, say, nattorei around, as well as old standards like Blissey and others, they seem fairly manageable for now.
