Sorry for the double post, but after a thorough look, here are my proposed changes to the Viability Rankings and the Skymin thoughts as promised
Azumarill: 1 -> 2
I'm pretty sure everyone agrees that Azumarill is no longer the dominant force that it once was. The loss of its partner-in-crime Jirachi really gutted its ability to sweep, with Amoonguss simply not cutting it as a redirector. It only makes sense that it drops a rank; however, it became so lost in irrelevance that I propose that it drop two ranks instead, relegated to tier 2 where its now niche ability to sweep rightfully fits. It's simply not relevant enough meta-wise for me to comfortably say that it's at the level of the likes of Talonflame, Kyurem-B, or Cress, all threats that have actually seen usage in the recent Cup and Grand Prix. It's not as generally viable, but its capability to sweep with the proper support and match-up should leave it a fit for Tier 2 still.
Sylveon: 1 -> 1.5
For the record, this isn't really a pure THALK hate nom lol. While I did feel that this ranking was justified at one point purely due to the dominance of THALK, at the present things are currently changing. It would be fair to say now that THALK's chokehold on the meta is now in a bit of jeopardy--Sylveon, if it wasn't part of this team, wouldn't really deserve 1 in my eyes. This is purely because it is not as splashable to teams as the genies or Amoong are. Putting Sylveon on teams requires more nuance and thought, itself lacking the universal application that the rest of my (proposed) tier 1 of MKang, Heatran, Amoong, and Genies have. These five have seen significant usage outside of the THALK framework while Sylveon hasn't, and with THALK now less reliable I believe it's time for it to slide.
Aegislash: 1.5 -> 2
This one was hard, tbh. I kept mulling over the fact that I really am proposing Aegislash, of all Pokemon, to be Tier 2, and with good reason: Steel as a whole is not as relevant as it once was, and common match-ups in the metagame right now leave a lot to be desired. It just feels too dicey of a Pokemon to use in the metagame right now as, while it can leverage its serviceable bulk, it often becomes victim to frequent losing positions in-game, with Heatran in particular being a really no-brainer pick among other threats that can damage Aegi in the meta, such as Mega Zard Y, Hoopa-U, and Landorus-T. A Dragon resistance is not as valuable while resistances to Fairy are easy to find in other ways, dampening Aegislash's value even more. If anything, the value of having Wide Guard will prove key, but it really isn't something that breaks the barrier for 1.5; to illustrate, one can only look at the top replays for XY Cup and Grand Prix Playoffs, where it made little mark. To be fair to it, I do consider it the top Pokemon in 2 if ever, if not the floor of 1.5.
Mega Diancie: 1.5 -> 2
Controversial nomination #2, I do feel that Mega Diancie is in a really awkward position in the metagame right now and it showed with its dismal showing in the recent tours. As a Pokemon, it has always been a bit quirky; its base 110 Speed leaves it smack dab in the middle of the critical 110 and 130 speed tiers, masking its average bulk and leaving it free to Diamond Storm a bunch, which always hit for damage that always felt like they could be doing more. It doesn't have the outright power of Mega Charizard Y or Mega Gardevoir, the Speed and utility of Mega Gengar, or the sheer universality of Mega Kangaskhan; it's basically a compromise of the spread damage and Speed among the Megas that can be played aggressively. However, base 110 simply isn't that crucial as it one was, and recent trends can make its life much harder. It always had problems with Landorus-T and Safety Goggles Thundurus, but the emergence of a much more aggressive Rain and builds that can shrug off Diamond Storms in SemiRoom make its life as an attacker too hard. While it's still capable of dominance, I feel that there are better choices right now.
Latios: 2 -> 3
Latios just doesn't have much going for it anymore--its Speed tier isn't relevant anymore, its STAB coverage is walled by the common Heatran, and its prey to pretty much every relevant faster Attacker (Talonflame, Mega Gengar) and Mega Kangaskhan. Literally no one used this except that one guy who used Glennn in this day and age. While Tailwind combined with the Speed tier is nice, there are more consistent and better options. Its viability is strikingly low, and it's also pretty irrelevant.
Keldeo: 2 -> 3
This is pretty much the same principle as Latios, to be honest. While beating Mega Kangaskhan is nice, it is pretty much royally walled by Amoonguss, and it's disarmed if not outright escorted to the next game by Thundurus. This was not used, though its ability to demolish both Heatran and Mega Kangaskhan is well noted; even still, there's only really specific builds in which this fits, itself not really providing much utility other than being a hitter. I'd mostly pass unless I have a Mega Gengar or something.
Hydreigon: 2 -> 1.5
Hydreigon was a victim of the shift to a more THALKed up metagame along with Latios, though unlike the Eon, Hydreigon was actually pretty poised to strike up the rankings. Sylveon's presence in the metagame was really the only thing that was keeping this guy down; otherwise Hydreigon is a pretty deserving Pokemon for 1.5. Its base 98 Speed, while short of tying with Mega Kangaskhan and Mega Charizard Y, still leaves it a predator to Kyurem-B and non-Scarf Landorus-T. Meanwhile, it has a stellar combination of defensive typing, bulk, offensive prowess, and utility in Tailwind that renders it a valuable piece in most cases. It can be pretty handy for switching in to the likes of Heatran and Volcanion, while its Dragon / Dark coverage leaves it capable of hitting everyone neutrally i.e. unlike Latios it's not damn walled. With Sylveon's clasp of the metagame shrinking, I believe Hydreigon is deserving of a promotion for the things that it can and always has been doing.
Diancie: 3 -> 2
Diancie is a surprisingly strong and splashable Pokemon if you're looking at building a SemiRoom team or two. Its considerable bulk and defensive typing render it capable of sponging neutral hits with ease and gives it some really handy match-ups; Mega Kangaskhan has to Seismic Toss to do significant damage which is a confirmed non-kill without assistance, while EP from Heatran and EQ from Landorus-T don't even come close to KOing. This isn't even counting the fact that foes are apprehensive of hitting it with a super-effective move that does not secure the kill, fearing the threat of Weakness Policy. On that note, Diancie is also an unseeming menace offensively, its Diamond Storms unable to be weakened by Intimidate thanks to Clear Body; in conjunction with the Defense boosts, it can surprisingly take over some games if left unchecked, especially if cheekily equipped with a Stone Plate or something. Diancie stands as a really strong pick for a Trick Room setter that does not want to give up any offensive pressure at all as, unlike Cresselia, its offensive presence is immediate; this fact also renders it a viable teammate for Amoonguss in SemiRoom squads, as having Cresselia and Amoonguss in one team can end up being too slow for some teams to swallow. It's very much a deserving Pokemon for Tier 2, and if it somehow makes it to 1.5 in the future I wouldn't even be surprised.
https://replay.pokemonshowdown.com/gen6doublesou-954669060
https://replay.pokemonshowdown.com/gen6doublesou-955264053
https://replay.pokemonshowdown.com/gen6doublesou-940978146
Ludicolo and Venusaur: UR -> 3
Weather teams, for some reason or another, have seen a bit of an evolution as of late. Rain has seen a trend of aggressively using more than one Swift Swimmer, and if one of them isn't Mega Swampert then Ludicolo becomes the sole viable choice. It has shown to be an effective Pokemon in these kinds of Rain, even starring on both teams in the XY match of the Grand Prix final. On the other hand, Venusaur, while definitely much more niche, has seen valid use as a lynchpin in stifling faster foes in
my Mega Charizard Y team
s. While of course not the best of the best, the use of these Pokemon are shown to be indeed valid, albeit niche, and thusly deserving of a lowly Tier 3 ranking.
Jellicent and Milotic: 3 -> UR
They are pretty irrelevant Water-types. Unlike the rest of Tier 3, which for some reason or another have a valid and distinct niche that would merit them being picked, these guys are just simply outclassed. There are better things to use.
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Re: Shaymin-Sky
I'm essentially going to echo
Stratos here and say that there absolutely should be consideration for dropping Shaymin-Sky back into the XY DOU metagame. Of course, while the way it got to DUbers in the first place is dubious at best, I'd suppose we focus on what Shaymin-S brings to the table now; while what happened back then was quite heinous administratively, what's done is done, and the metagame has changed and evolved.
It is of my tiering philosophy that, when we drop Pokemon from DUbers to DOU, what we should consider is not the possibility of it being a healthy addition to the metagame, but rather whether or not it can exist in the metagame without being unhealthy and thus deserving of being in DUbers in the first place. The rhetoric, while pragmatically similar, are different in essence, as instead of tackling why it fits in DOU, what I am presenting is reasons why a Pokemon should
not be in DUbers, my take of DUbers being a repository for Pokemon that bend the "health" of the standard metagame one way or another. Therefore, what is to assess here is the qualities of Shaymin-Sky and whether or not these render it a possibly unhealthy presence; I am not discounting what Stratos has presented, as they are also evidence of Skymin being "not unhealthy" through inference, but rather what I shall present is a different framing of a shared conclusion.
As I can remember it, Skymin wasn't actually particularly sinister as a threat; if anything could have been a measure of it being unhealthy, it would have been its flinch rate. For full transparency, this is core to the argument for why I voted to ban it back then, but my stance has since changed. Skymin, as a threat, is simply a fast attacker with a nifty speed tier, strong single-target STABs, average bulk, an a nifty ability to break a foe special defensively; nothing here actually screams broken to me, as it's pretty much a high-speed damage trader with a bonus. Is it strong? Probably, but not really something that I could reasonably say unhealthy.
In terms of fit to the meta, it doesn't even look like it'll break anything too hard. A slew of threats, such as Scarf Landorus-T, Thundurus, Shuca Tran, Talonflame, and rain... among others, can go toe-to-toe with it handily. By the very nature of how it is as an attacker, it'll also be a bit reliant on trades; if it wants to support with Tailwind, it loses out a coverage move, and even then a fast Tailwinder that's not damn Talonflame isn't gamebreaking at all. As Stratos mentioned, the old Sash set it used to run may not be as consistent as it was with SR now a more prominent factor, so it can't leverage that either. It's not going to warp the metagame around it nor is it gonna monopolize a style or role or something; based on my read on it, it'll just be...
there, which is honestly a good outcome if ever.
Its ability doesn't seem like it could break it, now that I think about it from a fresher perspective. Its ability to Seed Flare SpDef drop, while powerful, honestly seems like a pretty reasonable niche to me; it's not as if it would make noticeable differences every time when paired with already strong attackers, it only really being handy for breaking something resisted or something excessively bulky like a Cresselia. Air Slash flinches on the other hand... while luck-based, they can be taken to a point where they seem fair enough. There's an inherent opportunity cost with attacking due to the nature of being a single-target attacker, and its bulk renders this cost much more apparent; this is something that can be reasonably played around, as a matter of course if nothing else, without much fanfare. Meanwhile, both of its STABs also come at the cost of being inaccurate, and therefore unreliable and inconsistent; its nature as an attacker also relegates it to specific team compositions and prey to some others. While the nature of its being attacker has its potential, inherent and external factors will stifle Skymin's overt dominance, I believe.
Overall, I do feel that dropping Skymin to DOU should be voted on in some form. Of course, it would be unfair to do that without seeing its fit in the present environment (regardless of the fairness of it getting banned in the first place). What I propose is that we do tour / series of tours wherein Skymin is present in the XY DOU metagame. Things are different, Jirachi is banned, yadda yadda yadda; it's only due diligence on our part as players to experience such a metagame with Skymin in it before we assess it. The soonest tour we have that has XY on it is DPL (I think), so we have all the time in the world to do this. Hopefully the administration would consider, in appreciation of the game.