Okay, here are my thoughts on the current metagame.
Biggest Threats (Top 3)
Threat level measured in stars.
1. Manaphy-******
-Seriously, the Wacan berry set in the rain is insane. There are very few pokemon that can guarantee a OHKO on it, before it gets out of control. A example is that a CB Outrage from a Garchomp will very rarely OHKO 4hp Manaphy with out at least one layer of entry hazards. The only pokemon that I can think of off the top of my head are Choice Specs Latios, and Shaymin-s. If Manaphy is already at +3, then they can't really switch in safely. Latios can come in a +3 Surf or Grassnot and proceed to OHKO the 4 Hp versions with Draco meteor. Manaphy can easily get past most of its checks with Grassnot destroying bulky water types. Under screens, it's basically GG. Manaphy is by far the scariest pokemon ever in OU, almost comparable to Kyogre.
Ban Worthy: Without a doubt definitely yes.
2.
Darkrai-*****
Unless you have a sleeptalker, or something that's immune to sleep, then it basically renders one of your pokemon useless. After it proceeds to put one of your pokemon, it gets a free +2 with nasty plot, and to remind you, it has 135 base attack! Dark pulse and focus blast offer nearly prefect coverage, resisted by Toxicroak, whose as frail as heck, and Heracross. Blissey can also stop its sweep with Thunderwave, but Darkrai could have Substitute to evade that. Darkrai is somewhat manageable, because of the prevalence of all the new fighting types and Mach Punch. Darkrai is also bulky enough to take a hit or two (survives a +0 LO Doryuzu Earthquake). With the cost of power Darkrai can run a Chople berry to and get past its checks.
Ban Worthy: Yeah, its pretty scary
3.
Shaymin-S-*****
Its not such a huge threat to me team anymore, but this things rips teams apart. Shaymin-s outspeeds nearly all the metagame, and can flinch most of it to death. It has fairly decent bulk, but it has a Stealth rock weakness.
Seed flare makes switching in a difficult challenge. Registeel is noted, that it can easily switch in with out any fear. But seriously who is going to use Registeel, its set up bait. Shaymin-s can Leech seed registeel to make it even harder to difficult to beat. I'm glad that Shaymin-s checks manaphy, but it is clearly too strong.
Ban Worthy: Yes, it is just too much for OU.
Other Huge threats.
Deoxys-A/
Normal Deoxys-****
-If you don't have priority, or a scarfer then it will rip through your team easily. It also forces switches easily, and can set up a Sub to get past some of its checks.
Ban Worthy: Yes, it outspeeds nearly everything and wrecks it. Substitute makes it even worse.
Latios-****
130 base special attack and 110 speed are monstrous. A specs Draco meteor will wreck everything in the tier, with the exception of Blissey/Chansey. Even most Steel types will take a huge hit.
Ban Worthy: Yes, Draco meteor spammage is not fun at all.
Doryuuzu-****
-If it gets a chance to set up then its extremely hard to stop. After a Swords dance the only methods of getting past it is Priority moves such as Mach punch, Aqua Jet and Vaccuum Wave. Another way is to send out a very bulky pokemon, such as Hippowdon and Suicune. Bronzong and Skarmory don't really count because they can't really do much back. With a balloon it can easily get past it's checks, and with a chople berry, it can get past most other threats. Also of note, though gimmicky, Scarf Deoxys can easily OHKO with Superpower.
Ban Worthy: I'm leaning towards no, since priority users are very prevalent in this metagame, and weather changers make it much less threatening.
Poltioed-***
Poiltoed is no wear near as threatening as the other ones, but's its Drizzle that allows other pokemon like Manaphy to be beyond broken. Politioed can be quite powerful since it's water type attacks are boosted.
Ban Worthy: Yes, because it's the sole pokemon that makes Manaphy so overwhelming.
Wobbuffet-***
I haven't really seen many of these but they are certainly annoying. But they can guarantee a free set up turn, which can be breaking especially with Lucario, Shaymin-S, Doryuuzu, Manaphy, Darkrai, Garchomp, Ulgamoth, Cloyster.....and the list goes on. It also can easily remove walls with tickle, and revenge choice item users. The custap berry can also net a couple of kills.
Ban Worthy: Yes, it can shift the game into where your pokemon are literally massacring the opponents team.
Kingdra-****
With rain, it's a total beast. It can outspeed also dragons, even scarf Latios, and pound them into the ground with outrage. It can attack from both sides of the spectrum, and cause massive damage. There are few decent checks though, such as Empoleon and Natteorei.
Ban Worthy: Possibly, It's up to par with Doryuuzu or may even higher since it can be mixed and wreck everything.
Notable mentions:
Ditto-***
Pretty much the perfect revenge killer.
Ban Worthy: No, Substitute screws it up, and it is isnt really threatening unless it copies boosts.
Garchomp-****
With Sandstorm, it becomes almost as threatening as Doryuzu, it has more bulk and sand veil makes it tricky to counter.
Ban Worthy: Possibly, it is quite threatening since it outspeeds most things, and it has decent bulk, it can get past most of its checks.
Latias-***
Weaker than her brother, but it's still a threat, it can easily set up on more pokemon because it has great bulk.
Ban Worthy: No, it isnt nearly as threatening as the other threats. It is a great supporter and can be played offensively.
Mew-***
A great baton passer, and is very unpredictable. Can pass a +2 in just about every stat. Hypnosis, though unreliable can put a damper on its checks.
Ban Worthy: Leaning towards no, since it really only has one huge niche and that's baton passing.
Deoxys-S-***
Still a good spiker, though not as good as it was last generation. It can also be used a good revenge killer.
Ban Worthy: No, it really isnt very threatening, as there quite a few leads that will stop it from getting down at least 2 layers or spikes.
Salamence-****
It's still a huge threat, and can wreck teams with Draco meteor, Hydro Pump, Outrage, Fire blast. There arent really many good switchins. You just have to predict right to aviod it ripping your team to shreds.
Ban Worthy: Possibly, it is really dangerous and will wreck just any pokemon that switches in.
Kyurem-***
Horrible defensive typing, but it has a BST of 660, and it's ability doesnt hinder it. it's also very bulky and extremely destructive since it has 130 Base attack in each stat, and has decent speed, which though is just below many prominent threats. Also worth a mention, is that it really struggles with Steel types, having to resort to Focus blast and Hidden Power Fire.
Ban Worthy: No, it is really dangerous and will wreck just any pokemon that switches in. It is fairly manageable though, since it has a Stealth Rock weakness, and is outsped by most other dragons.
Other
Ninetailes-**
Sets up eternal sun, but really isn't very threatening, and has Stealth rock weakness.
Ban Worthy: Probably not, Sunny day teams arent nearly as threatening Rain and Sandstorm teams can be.
I'm not sure If they're DW Abilities are released but a few would be particularly threatening :
Blaziken-****
Shandera-***
Volbeat-***