Alright guys! We've kinda derailed a bit recently talking about whether or not top-tier types need a nerf and discussing the January stats, but I think we should go back to discussing our current suspects, then the up-next ones, and then decide whether or not to nerf these types, as we will probably have another month's stats to use, and it will also give people more educated opinions if they are actively considering this problem for a bit, rather than just saying ">10% needs nerf."
As for the suspects:
Mega Sableye:
On Dark I believe it needs a ban. It has tremendous support, from every angle. Wish passing and Heal Bell support is readily provided by Umbreon. Umbreon, Tyranitar, and Mandibuzz provide nice walls to switch in on attacks it doesn't like. Tyranitar, Hydreigon, Absol, Crawdaunt, Greninja, Bisharp, Houndoom, Honchkrow, Krookodile, Weavile, Drapion, Sharpedo, and Scrafty are all viable and provide tremendous offensive support to Mega Sableye, eliminating threats, checks, and counters, through wallbreaking, glass-cannons that can take out a Pokemon or two, other potential sweepers who can at the very least chop up the opposing team a bit, and multiple priority users. Heck, it even gets Zoroark mind games, and if you have it last in your party, then Zoroark can potentially eliminate your opponents go-to Mega Sableye check, or at least scout who it is and damage it a bit. It also has a few equally destructive sets. The most common moves will usually be Calm Mind and Recover, but it can then run WoW/Dark Pulse or Dazzling Gleam, Dazzling Ball, or even Taunt, Knock Off, Confuse Ray, Snarl, or Foul Play. It receives tremendous support and can sweep many teams when used effectively, and even if a counter can beat one set, it might lose to a coverage move, such as Dazzling Gleam, Snarl, or Confuse Ray hax. To conclude, BAN ON DARK.
On Ghost I think it is alright, so don't ban. Knock Off can really screw Ghost over, as can Dark teams, or random Shadow Ball coverage on something like Yanmega (I used to use that to have an easy time with Ghost--I used Metronome on it too to increase the damage, and it swept almost every Ghost team, usually from the start), Gardevoir, or Lati@s. Mega Sableye finally gives them a good answer to this, as it is neutral to both of their weaknesses, and can't have its item knocked off. Ghost also doesn't supply great defensive support. Some of its main walls are Dusclops (Knock Off weak), Dusknoir (single-Ghost typing is plain, and offers bad team synergy, no reliable recovery means Knock Off screws it too, as it relies on Lefties a lot), Gourgeist/Trevenant (weaknesses to both of Ghost's weaknesses as well as common coverage moves in Fire/Ice/Flying), and Jellicent (again weak to both of Ghost's weaknesses and common coverage in Electric/Grass). All of these have disadvantages. Ghost has no Wish passers, and the only Heal Beller is Mismagius, who sucks. Ghost actually does have decent offensive pressure in Gengar, Chandelure, and Aegislash, but that's about it. Mega Sableye actually helps Ghost become more viable and assists it in many type matchups, and because it assists in making Ghost a more competitive type in this metagame, it helps more than it hinders on Ghost, so, again, DON'T BAN ON GHOST.
Mega Medicham/Mega Gallade:
On Psychic, I believe they both need a ban. Mega Medicham offers tremendous walbreaking power, being able to 2HKO almost everything that isn't immune to or resists HJK. With coverage moves in Ice Punch, Poison Jab, and Psycho Cut, as well as priority in Fake Out (to break Sashes) and Bullet Punch (to finish off Sashed Pokemon and threaten Fairy types) it is an amazing wallbreaker. It has 100 base speed, which is average, but is enough for its job on Psychic, as it will outspeed everything it needs to kill (like Chansey, Porygon2, Skarmory, Ferrothorn, and Mandibuzz). Mega Gallade is an awesome sweeper. With incredible 165 base attack and access to Swords Dance or Bulk Up to boost that, it will hit hard. It also has a nifty base 110 speed tier, outspeeding most non-Scarfs, including Pokemon like Landorus, Thundurus-T, Terrakion, Keldeo, Infernape, and Garchomp, as well as the crowded base 100 speed tier and everything below it. With incredible STABs in Close Combat and Zen Headbutt, and then awesome coverage in Ice Punch, Thunder Punch, Knock Off, Shadow Sneak, or even SubPunch it can nail anything you want it to. Pokemon like Azumarill who may wall one set (in this case, it walls SD/Knock Off/CC/Ice Punch) are nailed hard by coverage (in this case, Thunder Punch, or even simply Zen Headbutt does a lot). It also has above average bulk to be able to set up. On their own, I actually don't think they are broken--just very powerful. It is the support Psychic provides that makes them broken. They get Heal Bell support from Meloetta, Mew, Gardevoir, Latias, or Celebi. They get Wish passing support from Gardevoir, Espeon, Jirachi, or Latias. They get Healing Wish support from Gardevoir, Latias, or Jirachi. They get offensive support from Victini, Metagross, Latios, Meloetta, Alakazam, Starmie, or Gallade (for Medicham only, obviously). They have access to bona fide walls to switch to in Slowbro, Meloetta, Cresselia, Deoxys-D, Jirachi, Mew, Gardevoir, or Latias. This is tremendous support, with amazing variety. Every one of the support categories contains at least 3 Pokemon, and usually more. When you combine this with what they already do extremely effectively, it makes them broken, as this support lets them perform their roles multiple times. For example, you can play recklessly with either Mega mid-game, before going to a Healing Wish user and bringing them back to full health to fight again. So, they are both fantastic Pokemon on their own, but then they receive absolutely fantastic support which pushes them over the top, BOTH MEGA-MEDICHAM AND MEGA-GALLADE NEED A BAN ON PSYCHIC.
On Fighting, I actually think neither is broken, so don't ban. They do of course receive amazing offensive pressure on Fighting, as it generally runs a Hyper-Offensive playstyle and has access to a variety of fast hard-hitters, including excellent Band/Specs/Scarf users. They don't, however, have access to any other support, really. Fighting has no Heal Beller, no Wish passer, no Healing Wish user, and below average walls (except Cobalion). Cobalion is actually a great physical wall that provides good support through Stealth Rock, Thunder Wave, and a slow Volt Switch to bring them in safely without having to sacrifice something. Their special walls however, are pretty much limited to Assault Vest Conkeldurr and Assault Vest Hitmontop. In my opinion, both serve their role without being too broken on Fighting, although I am slightly on the fence about Mega Gallade, because if their Scarfers or fast Pokemon are eliminated or Paralyzed, it can come in, set up, and sweep almost every type. This can be easy to accomplish with Fighting's own powerful Scarfers and Cobalion being able to tank a hit and Thunder Wave back. So, definitely DON'T BAN MEGA-MEDICHAM ON FIGHTING and MAYBE BAN MEGA-GALLADE ON FIGHTING. I am leaning towards no ban, but I could be convinced that it needs a ban.
TL; DR: See the bolded bits. Capitalized bolded bits are my main opinions.