Reuniclus got some extra attention in the past few weeks and in the beginning I was not really sure about its viability. It is a very nice pokemon on some teams, as it can set up on pokemon like Mega Diancie and win most CM wars bar Mega Slowbro, Unaware Clefable and Mega Sableye, but it is very difficult to fit on teams and this can be seen by its low usage. Because of its nice bulk and Magic Guard it can be a huge threat because nobody prepares for it, but it really needs some support because some pokemon that are common now can really give Reuniclus problems.
AV Slowking is pretty cool as it is a very solid switchin to many special attackers and can also catch people off-guard when they underestimate the bulk of Slowking. I use a more defensive build (Calm nature instead of Modest, but still with some SpA) and it is a very good switchin for Mega Gardevoir, Latios and can surprise LO Gengar when they are only doing 52% max. Besides the obvious Scald and Fire Blast there are many nice options such as Future Sight, Dragon Tail, Ice Beam, Psyshock and Power Gem. Especially Future Sight is pretty cool as Slowking is easily forced out by physical attackers and status, and giving them some extra pressure with Future Sight is very good. Also stops some pokemon with Recovery like Mega Altaria from winning the Roost / Attack war.
I don't really see much reason to still use Mega Medicham. It is a good wallbreaker but the standard balanced builds can handle Mega Medicham pretty well. Pokemon like Kyurem-B, Pangoro and Tornadus-T can tear these cores apart much easier (not saying Mega Medicham is a worse wallbreaker, only saying that defensive builds nowadays can handle Medicham much easier than they can handle Kyurem-B) Against stall it is still very good because stall doesn't prepare that much for Medicham anymore, but this only applies when the stall team doesn't run Mega Sableye, who just stops Medicham (Foresight Mega Medicham New Meta?)
Adamant Band is another set that can give problems to defensive cores. V-Create does a lot even to those who resist it (up to 55% to rotom-w for example) Another set that has much usage is LO Glaciate, which can surprise pokemon such as Rocky Garchomp and Lando-T who try to force you out with Earthquake after a V-Create drop. The speed drop is also very good as a team supporter, while also taking care of scarfers that want to switch in. The problem with Victini is that it doesn't fit that well in a metagame were sand offense, and in particular Pursuit Tyranitar, Mega Altaria and Keldeo belong to the biggest threats.
Fuck this thing. Not even worth discussing. It is said Gothitelle only works against stall but I disagree with this. Gothitelle fits on some teams as someone who traps pokemon like Chansey who stops Zard-Y or Clefable who stops Kyurem-B from sweeping the stall team, but it is also a pain to face for some offensive builds because it can Trick a Scarf to pokemon like Heatran, Tornadus-T and Landorus and while they are not defeated, they lost their niche to change moves and this gives the teammates of Gothitelle a free turn to set up and make it easier to switch into the choiced pokemon. It is also found on stall teams, though it gives very bad defensive synergy, as someone who stops Manaphy, Clefable and Diggersby from sweeping. Especially more aggressive stall teams really appreciate the help of Gothitelle.
This thing is a big threat for offensive teams if used as a late game cleaner.It is stopped by Mega Sableye and Chansey (lol knock off) so against stall it is really difficult to put in work, but even fast offensive teams may lack pokemon that outspeed Mega Alakazam, as it outspeeds pokemon as Mega Beedrill, Scarftar and +1 Mega Altaria. I have to say it is pretty hard to use it right because of its low bulk and (even with 175 SpA) difficulty to OHKO pokemon it doesn't hit super effective. It also has to watch out for Bisharp's 50/50's and basically everything that outspeeds it
This thing has 352 sets and they all have a cool niche in the metagame. SpDef is a very good switchin to pokemon like Kyurem-Black, Mega Diancie and Mega Gardevoir, which are often big threats for stall teams. The scarfed set can also take care of these threats, but is only a one time switch in and is more of a momentum grabber. Offensive sets with Stealth Rock and coverage moves can really surprise some opponents and apparently there is a set called ParaFlinch that is so good that it let you win without even using skill, it is just that good.
This thing was awesome the 2 weeks before the mega metagross suspect was announced because it could (and still can) take care of threats like Keldeo, Mega Metagross (needs mixed def for Keldeo and GK Megagross) Landorus-T, Mega Lopunny and Talonflame. Even if it predicts wrong it can use Regenerator to stay healthy. It has tons of good moves like Thunder Wave and Scald which supports the whole team. The reason it is a bit less effective now is because of the rise in usage of Kyurem-Black, LO Starmie and Tornadus-T, who all have ways to defeat it.
Still a very good wallbreaker, but the fact Mega Metagross didn't get banned, Victini and Jirachi rose in usage and Hyper Voice is in general a little less spammable than before really hurt Mega Gardevoir. Though I have to say the metagame is not very prepared for it so you can really smack through some common builds.
This thing was overrated af in the beginning of ORAS, but the metagame prepared for it a lot and it decreased in viability. At the moment I think it is a little bit underrated for it capacity as a very good wallbreaker and set up sweeper. Again, the metagame doesn't really prepare for this threat that much
anymore so it can easily get some kills if you don't have a solid check. There is still Mega Sableye, Tornadus-T, Clefable etc. but there is also a decrease in usage of Talonflame, DD Altaria (more special sets now) and Thunder Wave Thundurus, which is beneficial for Gallade.
I play a lot of OU matches and Mega Slowbro is a very interesting case. As we all know, Mega Slowbro was one of the most overhyped mega's at the start of ORAS, tied with Mega Swampert and Mega Gallade as most overhyped, and the metagame get used to it pretty quickly, with pokemon like Thundurus getting a lot of usage. With the ban of Greninja, Mega Slowbro's viability grew even more (A to A+ in viability rankings) but Mega Slowbro's usage is still a bit low. (3,07% of the OU teams has Slowbro as mega. Meanwhile Scizor, Sableye and Metagross have more than 9% usage). The metagame evolved a bit and I mentioned a few problems Slowbro has now but they don't all apply for Mega Slowbro. Mega Slowbro doesn't have a Knock Off weakness, it can run a bit spdef because 16+ defense is enough to live strong physical attacks and it runs a CM set much better. Many teams prepared for CroBro and Slack Off CM Slowbro with reserving two mons who both could beat one of the sets. Examples: Unaware CM Clefable + Toxic Chansey, Taunt Thundurus + SD Scizor and Magma Storm Heatran + NP Celebi, which was not only used for Slowbro, but still got Slowbro to struggle against these teams. Not only is there a little decrease in the usage of these sets and some of the pokemon (Thundurus, Toxicroak) but there is not a combination of these, because the wallbreakers that are common now give these cores problems. I feel like the metagame is evolving into a metagame that is very favorable for Slowbro, especially for the Slack Off + CM + 2 Atks set. Ofcourse the metagame has to develop more but as it seems now Mega Slowbro becomes a bigger threat every day.