I may be repeating what others may have said, but here is my take on some of the candidates for testing/banning in the near future:
Inconsistent needs to be the first thing on the chopping block. From what I have seen inconsistent, while not completely game-breakingly good all the time, adds an unnecessary level of luck to the metagame. Additionally, the best you can usually do is force the inconsistent user out so it can try again later by using perish song, whirlwind, roar, or maybe haze/clear smog. Even that is usually detrimental, as you use up a turn while taking an attack to the face. Other than that, you have to hope for speed and evasion drops so you can actually attack/status it. In a nutshell, the ability inconsistent is a pain in the you-know-what, and it usually adds a huge element of luck to winning battles.
Skymin is annoying at times, but I am not completely convinced yet that it is broken. Skymin is blazingly fast for the metagame's standards, outspeeding all commonly seen pokémon without a boost except deoxys-A and deoxys-S. That killer ability serene grace is downright frightening considering it can wield seed flare and air slash to great effect. It has excellent type coverage with these moves and earth power or HP ground. It is also one of the best subseeders in the game with its reasonable bulk and blistering speed. However, skymin is not without its weaknesses. An obvious drawback is its bad defensive typing, being notably weak to rock, fire, and ice (4x), very common offensive types. It is also outsped by scarfed pokes with a base speed of 70 or higher, practically anything with +2 speed (like from chlorophyll, etc.), and those with access to priority moves (including erufuun's crippling stun spore). Another note is that air slash and seed flare have a nasty tendency to miss when skymin does not want a miss. All in all, skymin is threatening, but generally manageable.
For me, deoxys-A was somewhat underwhelming. That is almost certainly because I made my laddering team to be able to deal with it, but still. It is brought to shame by anything with good bulk and an offensive move. On the other hand, you really do have to play around it because it is very dangerous when allowed to attack more than once or twice. Extremespeed is a great revenge-killing move, but superpower and psycho boost are what make its offensive prowess known. Once again, I must stress how pathetic its defenses are. It has less defensive ability than smeargle! Every move sans rapid spin that hits will probably be an OHKO. Forretress has been invaluable in this regard, being able to withstand deoxys-A’s attacks with ease and counter with earthquake or gyro ball. Deoxys-A is easily revenge killed with priority, and extremespeed will rarely KO you first. The other deoxys formes are not nearly as immediately threatening (except maybe Deoxys-N, but I have yet to see one of those).
I have had really no problem at all with darkrai so far. Dark void is now at 80% accuracy, making it a bit shakier than it was last generation shaky. Sleep clause greatly limits its use anyway. Dark is a nice offensive type, but still has to rely on the notorious focus blast if it wants that much-needed power and coverage. Speaking of power, darkrai has access to nasty plot, and it has the stats to exploit it. Unfortunately, it has to make do with the 80 power dark pulse, though it has that nice flinch chance. Darkrai can also make good use of a trick scarf set, foiling many sleep absorbers and random counters that switch in. Darkrai, like skymin, is very fast. Base 125 is very good, but priority fighting moves are a problem without a custap berry. Its defenses aren’t abysmal, but darkrai just doesn’t enjoy taking hits at all.
I’ll talk about weather-related threats when I get the time later on.
Edit: Dark void's accuracy was always 80%. For some reason I thought it was 90% last gen.