Of all the viable ban options, there are only 4 which are able to be considered broken, and even out of those only 2 are really, really broken.
Inconsistent-This is simply unfair and makes for not only shitty games, but a shitty tier. Barring absolutely awful luck or an unwinnable matchup (like octo vs specs jolteon), the octo is going to set up, get comfy, and wreck havoc. Running boil over/protect/sub/rest makes it even deadlier, as you gain survivability with rest. I just had a 100+ turn battle where my octo stalled out a CM/recover latios and then swept the rest of the team. Seriously, it needs out.
Skymin-How broken this is depends on how you choose to play it. I've only seen 4-5 different sets out there, mainly scarf, specs, lo, and sub leech, and out of all of these, scarf is the most uber. Base 125 SpA is serious business, and with flinch hax+seed flare, this thing can seriously take down almost anything. Scarf skymin is even fast enough to outspeed dorys and kingdras if they don't run max speed, in their respective weather boost of course. On top of that, skymin isn't killed by priority that well either. Even CB scizor barely does 60-70%, which means that if it switches into an air slash its dead and if the skymin uses hidden power fire its dead. Overall a nasty beast.
Darkrai-Eh, I thought I'd be more impressed. I run a scarf skymin, so thats one possibility why darkrai doesnt threaten me at all: Skymin is able to almost completely shut down darkrai, due to that lovely 127 speed. Still, when skymin is out of the question, darkrai really only has 4 viable attacks it can use in dark pulse, focus blast, t-bolt and ice beam, and while that affords it excellent coverage, it can't run them all at the same time and none are overly impressive on their own. Dark void, while 80% sleep does rule, is prone to missing at key circumstances, and when it misses it usually goes very poorly. All this being said, I still give this a slight uber nod, simply because it does have very high base SpA, and is one of the best non set up sweepers in the game.
Deoxys/Deoxys-A - Like darkrai, these fall into the "countered by scarf skymin" which makes them far more manageable, but I'll look past that. Deoxys-A is a true powerhouse, capable of serious damage. While his paper defenses mean virtually no switch ins, this guy can run a LO set and pose a serious revenge threat everytime a pokemon is downed. A good comparison to Deoxys-A would be the grenade drop perk in Call of Duty, where upon dying you drop a grenade to kill your killer. Deoxys-A works in a similar fashion, coming in after something dies to make you regret it. While not impossible to work around, it is simply too strong for OU.
I know there are other potential suspects (Mew, Wobby, but not Manaphy. There is nothing broke about Manaphy), but in all of my battles thus far, the only one which actually changed the outcome was a Heart Swap manaphy, who stopped my octo sweep, just to reinforce that inconsistent is so broken it takes heart swap to stop it. Mew has a lot of potential, he just doesn't fit in well with the weather teams which dominate the land, and I think all the Wobby players I have played are just ignorant to his strengths, because they have been using him awfully (though teamviewer does hurt Wobby, a lot). I may or may not be in voting range, but here are my opinions.