Ok, there's going to be all the usuals here, then a few things to think about later. I'll start with the easy ones.
Deoxys-D: This thing was banned for its combination with a defiant user and ghost-type. It's not dual-typed so it's weak to all the things the team's weak to, and there's plenty of good defensive mons for psychic to choose from like slowbro and AV meloetta. The offensive psychic pokemon can use its hazard support, but this thing certainly doesn't seem to make the metagame unballanced, and we should only ban where there is a definite reason to in order to make the metagame more diverse.
Don't ban.
Deoxys-S: Similarly to Deoxys-D, this thing was largely banned for hazard support, and while it can run a good offensive set or run dual screens, it simply doesn't seem that OP when considering it's weak to the same things as the rest of its team, and so doesn't provide particularly much defensive support, while there are already dual screeners and powerful attackers such as scarf gardevoir.
Don't ban.
Genesect: Of the three big unbans, this was perhaps the one people dreaded least. A week later, it's the one I see most concensus on. Almost everyone I have spoken to about this wants it banned, for a number of reasons. Firstly, it's being given to teams that are already strong. Secondly, it creates a huge amount of momentum, and a large number of 50/50s where if the opposing player gets it wrong they lose a pokemon, but if they get it right they deal 30% to a switch-in. And finally, it's powerful enough to be a massive threat in itself. My first piece of evidence for this is that I had to change my Landorus-i set to having 10 defense IVs, otherwise I pracitcally auto-lost to this thing as soon as it got a free switch-in, with ice beam KOing at least one of my pokemon whatever I did. My second piece of evidence would be Auburn's post here:
I do not see that any more evidence of it being broken is needed: For example, in Auburn’s game against All Falls Down, I would argue that AFD was playing better and were genesect not there would also have won, which is certainly what should happen in a competitive game: The better player winning despite steel having a mild type advantage.
Ban.
Aegislash: I don't think this thing is at all broken on ghost teams. On steel teams it was a horror, as between Heatran, Skarmory and Aegislash they were immune to each of the types SE to steel. Ghost, however, needed a versatile pokemon that was strong but counterable, and this is exactly what Aegislash seems to be on ghost teams. With a defensive set stally ghost becomes just about viable, and with an offensive set offensive ghost becomes viable. This was a perfect example of how Monotype breaking off from OU was great for the meta, Praise Nani and Praise Monotyke!
Don't ban.
Kyurem-White: I will admit that when I first saw this was being unleashed onto the Monotype metagame, I didn't know what was going on. It's extremely powerful, and incomparable to Kyu-B due to its far better movepool. While Ice-types don't provide the best support, they can provide enough through Rapid Spin, Stealth Rock, and a little walling / pivoting. The problem is that the way to beat it isn't to attack it, but to take out its support, and a small slip in this can be more than enough to lead to a sweep,
as this replay shows. Sure, BrendanLee should have had a revenge killer, but even if he had used, say, scarf thundurus-t with focus blast, Jak could simply have sacked something to focus miss, sent in an ice shard user, double switched to Kyu-W as the revenge killer switches out, and repeated the process. It's not that it's unbeatable; if BrendanLess predicted correctly in that situation then he might win, but focus blast is barely accurate despite Thundurus-t being one of flying's better revenge killers for the monster, and it stops being a game where teams are involved and starts being a battle between two pokemon with a little support at the start.
We also need to make sure that in trying to make one team stronger, we don't make the metagame unbalanced by causing other types to fall. Let's not forget that the first step we took in becoming an independant tier was banning talonflame for making matchups against grass, bug and fighting too one-sided. I can't help but think of matchups Kyu-W causes to be one-sided here, grass once again being on the list but so too are poison, flying, and ground (252+ SpA Choice Specs Turboblaze Kyurem-W Ice Beam vs. 252 HP / 252+ SpD Gastrodon: 243-286 (57 - 67.1%) -- guaranteed 2HKO after Leftovers recovery)
In the middle of all the Kyu-W debate let's not forget why it was considered in the first place. Ice has problems because it's generally a weak type, sure, but the main problem was the lack of ways in which it could deal with Mega Scizor. And what do you know? It still can't. Kyurem-White may be reasonably good against Steel teams, but if Mega Scizor gets to +2 and there's Stealth Rock up, it sweeps same as it ever did. The main problem ice types had is still the main problem ice types have, and their only way to win against mega scizor that they didn't have before is to sweep before steel gets the chance. Once again it's a battle of two pokemon with some teams on the sides, rather than a tactical game featuring 12 independant pokemon, where support pokemon may be considered just as important as sweepers.
And as was eventually decided in
this thread I keep spamming a link to, the true purpose of the metagame is to be fun. While Kyurem-White makes ice usable, it makes the game far more matchup-based, far more centered around a single pokemon, less competitive and in my opinion, well... less fun. I'm not gonna use bold when I write Ban because I want people to have read this. It's important to me and important, I think, to the future of the metagame.
Skymin: While this pokemon, given good support, is overpowered, it isn't as powerful as Kyurem-W nor as unpredictable as Genesect. I'm not exactly a good grass player but I played around with it a little, and it's very powerful. It gets plenty of entry hazards from ferro, defog from shiftry, healing wish or baton passed boosts from celebi, and of course Mega Venusaur walls many of the things grass dislikes. It causes fighting, water, ground, rock, bug, and opposing grass not using skymin to struggle but at the same time, it doesn't help all that much against the things grass has always struggled with: ice, steel, flying, fire. It also forces the grass player to make a far more supportive team, as a normal grass team would generally be a lot more bulky and less capable of keeping up offensive pressure, while at the same time Skymin doesn't add any resistances of note in the same way that Mega Venu can tank ice and fire moves, or ferrothorn can take flying, poison or bug moves and shrug them off.
So overall, it doesn't actually seem too powerful. It is, however, bad for the metagame as it requires grass teams to change dramatically if it's to be used well, causing more of this "One pokemon with a team supporting it" mentality rather than a core or whole team working well together. It also causes the games to be far more matchup-based, which is uncompetitive and less fun. So while it wouldn't be gamebreaking if it weren't banned, I'd still prefer a
ban.
Mawilite: Ok this is going to look like a cop-out given the essays I've given about other mons, but honestly I haven't seen too much of Mega Mawile. My games against steel have generally been me vs. genesect, though I guess I've had to keep up the pressure on mawile by keeping Zard-X or Landorus in, or having skarmory healthy in the back. Fairy teams, on the other hand, seem to have been scared off by the large number of steel teams infesting the ladder as I've barely seen any since the unbans. At the same time I guess most of my thinking has been on other, more important mons to the metagame. Honestly though I don't know if this should be banned or remain unbanned, but I trust Nani's decision on the matter either way.
Now for some interesting things.
Talonflame: People have brought forwards Talonflame, saying that if we're unbanning Kyu-W, Skymin and Genesect then Talonflame on fire should be unbanned as well. However I think this highlights the problems with the Skymin, Kyu-W and Genesect unbans in that Talonflame is no less broken than it was when we banned it, and the fact it's comparable to these new pokemon simply means they're broken as well. The ability to make games against certain teams very close to a certain win is uncompetitive, and so it is far better to
Ban these threats.
Zekrom: Someone brought this up for discussion, saying Electric was a weak typing. However, electric is a decent enough typing, Zekrom is OP, and allowing this into the metagame wouldn't particularly patch up Electric's current weaknesses, rather giving it more power. Electric has got physical attackers, Electric has got ways of dealing with Mega Venusaur, and the metagame would suffer if this thing were unleashed upon it.
Keep banned.
Mega Medicham: I seem to remember someone arguing that Kyurem-W should remain unbanned for ice because ice struggles with certain types loads. Well, let's take the problem to its source. I'm not saying this thing should be banned, but we should have a look at it and properly discuss it. It's very strong vs. ice, and there's this:
Moreover, Normal teams famously have a hard time with this mon, as physically defensive intimidate staraptor (their best option against it) is 2HKO’d. Normal, at least, has the option of running protect on a random mon and letting it crash and hurt itself, but this is barely a reliable counter.