Just some thoughts on the Deoxys formes (though I am guessing that many people have already thought along the same lines):
Deoxys-S - Ban
This thing is so versatile that it is not even funny. It is capable of running almost every other set ranging from (the list is not exhaustive) Sash hazard setter, Life Orb revenge killer, even Light Clay dual screens+rocks+taunt, or even a mix between different sets, each set able to carry its own weight (or more). There isn't even a need to go into detail on how great its 101 sets are, the fact of Deoxys-S's reliability is common sense to anyone with basic knowledge of competitive battling. Now, this also poses another unique disadvantage to the opponent. Which set does the Deoxys run? Is it the hazard setter of the opposing team? Is it going to be the lead or not? These are the issues that anyone going against a team with Deoxys-S has to deal with, which alone puts them at a big disadvantage. The person could very well feint a lead with Deoxys-S, only for him to send out something like Landorus-T on maybe a predicted Bisharp or Aegislash, and Deoxys-S could simply come in later in the game and pull of its job. Even if Deoxys-S is the lead, the issue of "what set does it freaking run?" causes another big headache anyway. For all you know, it could be a screener setting up 8 turns worth of dual screens in front of your Bisharp as you go for the Sucker Punch (paving the way for a setup sweeper to destroy at least 2-3 of your pokes), or even a Life Orb variant with Superpower for all you know. Point is, when facing Deoxys-s, it is almost impossible to know what set it is running (unless you played that fellow before), and you are just forced to blindly guess and hope for the best. Heck, Deoxys-s can’t even be counterteamed technically since there is no one pokemon that can counter all of its sets, and unless you played your opponent before, you can’t exactly predict what set is being used (unless you just want to counterteam a few players in question, which would mean you lose out more in general). As a result, more often that not, the person that is using Deoxys-S is effectively has the momentum of the game right in his/her hands.
Now, even if you successfully predict the set your opponent is using, it does not mean that Deoxys-S is not able to perform its duty. Screens and hazards can still be set (with or without some double switching depending on the situation), revenge killer variants still are able to place offensive pressure as long as they aren’t KOed. It still doesn’t change a thing for your opponent. The Deoxys is not going to lose its utility just because you might have figured it out. Barring your opponent being “unintelligent” or just plainly being Captain Obvious, the only possible change is that you wrongly predict the set and let your opponent have an easier time.
In addition of the element of surprise, as what people before me have highlighted, another potential advantage lies in the fact that it is capable of weakening what is supposed to be your check against certain threats, which makes you more prone. For instance, you might rely on Aegislash as a check to Thundurus especially if your team contains stuff like Landorus/ Gyarados, forcing Thundurus to switch out and back in on rocks (assuming you placed them), weakening it enough to prevent a sweep. Now, with your Aegislash taking a Knock Off from Deoxys-S, I highly doubt it can tank hits very well after that.
While it can be debated that since Deoxys-S can be used by anyone, it’s their fault that they didn’t run Deoxys-S. However, a pokemon that is top notch in performing multiple roles that basically eliminates competition for team supporters is something that is over-centralizing the metagame, which equates to a reduction in competitiveness anyway. Moreover, providing zero synergy in terms of defense, it is mainly gives offensive/hyper-offensive teams an unfair advantage since you can’t expect Deoxys-S to be run on Stall or most Balanced teams in the first place. I honestly don’t think that something like that should remain in the OU metagame.
Deoxys D – Ban
Although it is way more predictable in terms of its usage (discounting a couple of geniuses I met using a scarf Focus Blast set), the level support that it provides for hyper-offense is just ridiculous. Usually being DeoSharp + 4 more fast offensive users that can either Taunt or hit extremely hard or a team built around the same concept, it just makes slow moving Defog users experiencing living hell when trying to remove the hazards. Their options are: to risk being taunted and severely losing momentum, defog and give Bisharp +2 attack to make ensure that teammates will buckle under Knock Off/Iron Head/Sucker Punch, or simply defog and get KOed, with Deoxys-D being able to simply come in and set up once again. And this is assuming that the opponent doesn’t even try to double switch, forcing the defog users to come into the hazards multiple times and risk getting KOed even before they can defog. Even Excadrill has to predict correctly multiple times to be able to get rid of the famous Air Balloon Aegislash in order to simply remove hazards that Deoxys-D can just replace. Weigh in the fact that one wrong prediction can cause Excadrill to die in vain or force the game’s momentum to be relinquished over to the opponent, it just tilts the advantage way over in favor of the Deoxys-D team. In any case, the hazards are going to matter significantly. Sure, prediction goes both ways regardless of what countermeasure is adopted for hazards. Definitely, your chance of winning this mind game is not zero. It is just that your opponent has a way higher chance usually to win since the odds are stacked against you.
One might argue that fast defog users, with the Lati twins being prime examples, can help to even things out. While it is true, (beware of Pursuit using Bisharp though) you cannot expect to argue that every team is supposed to carry a Lati for this purpose, especially when a more suitable teammate depending on the team composition could be used instead.
Moreover, other checks/counters to Deoxys-D, such as Bisharp (ironically), are more often that not, only able to fit naturally on offensive teams itself. Once again, this forces defensive/balanced teams to fight an uphill battle.
Of course, if you use an offensive playstyle / like a metagame characterized as offense/hyper-offense, naturally both Deoxys formes wouldn’t seem overbearing. Even then, in terms of offensive support, they do over-centralize the face of that role in a way which is not exactly the most healthy for the metagame. Then again, do take into consideration on how other playstyles have their viability severely limited as a result.
Having said so, I understand that some might disagree, having alternate views. I have no intention in making this a “I am right and you are wrong, so you have to agree with me” kind of thing. This is just an opinion, feel free to disagree with it to the max.