Viability shifts all the time, so this is a snapshot of where I feel the metagame is as of last December. I'll share my thoughts on some of the higher ranked mons and on how I organize the viability tiers. I made this ranking by reviewing replays from my 1500-1600 rank games to analyze which mons had the most impactful role in each game, both on my side and the opponent's. While I think VRs are a tiny bit silly, since really they're a theoretical discussion based on anecdotal evidence, I think they're also kind of fun. Since I'll probably go inactive in the format again soon, I wanted to organize my thoughts about this moment in time, which I think is fairly distinct from the last time I made a VR for gen 4 a few years ago.
S1 Rank
Jirachi Breloom Gyarados
These 3 rarely go a game without doing something, and in about 1 in 10 games they just win the whole game by themselves. Each of these 3 simultaneously have insane defensive utility AND provide a ton of offensive pressure. They also have several possible sets, leaving a lot of questions for what you should do while facing them, meaning they can get away with stuff by bluffing coverage moves/held berries/EVs. I think it's pretty clear Jirachi is the centerpiece of the meta, so there's not much to say there besides that it has 11 possible moves, paralysis spreading like crazy, and a button that can win you the game with RNG. Loom's Spore though is the best click in the game, since it can incapacitate an opposing mon and set up something like SubSeedPunch which ruins offensive teams. Having a possible Mach Punch also reins in a lot of random threats. Gyarados is the best boosting sweeper by far, and Wacan/Lum can make it really hard to prevent from getting +2. Plus it has Taunt to prevent (p)hazing, and its Waterfall flinch can win you the game with a random 1 in 5 check. Defensively, Gyarados is really incredible, because even with SR and Sand it can check stuff like Infernape or Lucario or Heatran, but honestly, on a purely theoretical level I think Gyarados would be the best mon in the game if sand and SR didn't exist in this format. Of course, both of those are fairly likely to exist in any given game and require support to prevent, so I'm putting Gyara 3rd because I think the SR and sand weaknesses are a tiny liability, but it's very close between all 3 in this tier and you could argue any as #1.
S2 Rank
Starmie Lucario
Starmie is one mon I'm ranking most from my own use, because I haven't had too much trouble with opposing Starmie, but since I use Life Orb Starmie on nearly half my teams, I find it is really challenging for any of my opponents to handle. Unless you commit to using something very passive like Blissey or Clefable, it's a real nightmare to switch into. With SR up, Hydro Pump into Thunderbolt or Ice Beam takes out about any switch-in, meaning you can kind of spam it while outspeeding everything. Very importantly, Pursuit users can't switch into it all on Hydro Pump. It's also fast enough to completely clean up late game, and it has Natural Cure and Recover available to outlast special walls until late game or when Clef is below 75%. The worst Clefable can do is knock it off, and it still poses a threat without LO. Also it's the best spinner in the tier, since nothing can really prevent it from spinning without risking dying immediately. As for Lucario, a lot of my games have ended with a speed tie check between two Lucarios, and I've had a lot of games with Lucario sweeping through 4 or 5 mons to win with very little support. He is strongly resistant to SR and sand, very hard to wear down, and perhaps most importantly, he abuses the necessity of choice locked Pursuit on stuff like Scarf TTar very well. Also having an Espeed click in the back can save you from a losing scenario against stuff like a chipped +2 DD gyarados that's out of control, or really any offensive threat except DD TTar of Sub Gengar, which you could also hit with BP if you give up Crunch. But yeah, I would say both of these two match up well very vs offense, and also give stall a number of issues, especially in the current meta with Blissey being run less in favor of Clefable.
S3 Rank
Gengar Tyranitar Latias
I think these 3 are all a little bit more inconsistent in their impact in a game, but are all undeniably top tier. SubSplit Gengar is nearly impossible for stall to get rid of without running Blissey, and it also matches up really well vs offense. My main issue with Gengar is its reliance on a terrible inaccurate move, and if it learned Aura Sphere I would put it one tier above, next to Starmie and Lucario. The main reason I have TTar this high is because there are so many important targets for scarf TTar to trap (Starmie, Gengar, Latias), but I think the DD sets with random berries to help setup like Passho or Shuca or Lum are pretty impactful too - kind of like Gyarados, but a bit worse. I've seen a lot of games where Scarf TTar was the only way to handle a runaway Starmie/Gengar/Latias, but I also have seen a lot of games completely lost to scarf TTar providing a free sweeping opportunity to Breloom or Lucario or Infernape, so I think TTar is hard to rank, but around here feels right. Latias is worse as an offensive threat than offensive Starmie and Gengar imo. Part of the reason I think that is that it loses to them 1v1 (assuming HP Fire, if no HP Fire it can speed tie Gengar), but I also think clicking D Meteor as Specs Latias is less likely to make progress than clicking Hydro Pump as Starmie or Sub as Gengar. However, Latias has some nice bulk which lets it switch in a bit, and it has some tricky options with Trick and Healing Wish that make it stand out enough to still be considered top tier. I think a lot of the reason I am putting it this low is due to meta shift. Something like Latias is not going to do well with defensive Jirachi and TTar everywhere. I think it's just a bit outclassed and the mons ranked above it do a bit better in the matches I've experienced. The top 8 is very close imo, and I think Latias is in practice not really a worse mon to use than Gengar or even Jirachi, but this is just how I feel based on this current snapshot of the metagame I played recently.
A1 Rank
Bronzong Heatran Rotom-A Forretress
I would say these are consistently valuable mons, but perhaps fill a more supporting role than a game-winning role. For the first two in high tier, Zong and Tran are the best defensive steels after Jirachi since they don't get removed automatically by Magnezone. They can both switch into stuff like Latias or Jirachi and get SR up, trade blows, maybe eventually explode on a lure. Trick Room Zong should not be slept on either, especially in a format where most mons dump as many EVs as they can into speed. Heatran also can do stuff with Taunt. Zong can lure Heatran or Infernape with Heatproof, Tran can run Passho or Shuca to lure things. Overall they are both highly versatile supporters that also tend to set up SR and additionally get a KO or two in a game. Zong struggles with mons like Forre, Skarm, and pdef Rotom-A, and Heatran has some awful weaknesses in Fighting Water and Ground, so I wouldn't put them in top tier, but their strengths make up for their weaknesses in most team matchups. Rotom-A is a great scarfer, and it's your best bet at spinblocking in this tier, which is enormously important because of how impactful SR is. Forre appearing threatens spin, which is very important since it can force a switch into a spinblocker. Since it can just set up spikes when you try to switch in to spinblock, it's very good at creating pressure against stall teams, since defensive Rotom-A usually can't even 2HKO Forre after switching in hoping to block spin. It also of course can explode which stops it from being too passive vs offense. However, it is a bit specially weak and it can be trapped by Magne, so I'd say it is barely in this tier.
A2 Rank
Dragonite Azelf Skarmory Magnezone Metagross
Dragonite is like Gyarados in that SR and Sand make it sad, but it matches up really well into most teams. CBNite for example is able to harass passive teams really well, and having a huge CB Espeed click in the back is a solid win condition against offensive teams. I think Dnite suffers from Steel spam due to Latias and that's kind of a meta relational issue, but it is really good still. Azelf is a very versatile and powerful lead, not much else to say there. Skarm is Magnezone weak but otherwise a good hazard setter. Skarm can't do the physical walling by itself in this format due to stuff like Taunt Gyara and SD Lucario, but it does catch powerful hits from bit threats like Dnite and Metagross pretty well. Speaking of, Magnezone tends to either win games on the spot or perform fairly moderately, so it's hard to rank, but I'll have it here. Metagross has the coveted SR+Boom, and it's just behind Zong and Tran for me because its ability is less useful, and it lacks the versatility of having Trick Room or Taunt as X factor. I suppose its advantage is having the strongest explosion click, but it's less defensively useful and less versatile.
A3 Rank
Suicune Scizor Zapdos Gliscor Empoleon Celebi Infernape
I'll stop talking about every individual mon at this point, but this is my idea of the bottom of high tier. I think these are all very consistent Pokémon that will usually accomplish progress for your team in a game while also playing a defensive role, and not providing an exploitable position for your opponent to consistently abuse. As such, these mons don't feel like a liability on a team. They can also win games with a respectable offensive presence and ambiguous moveset. It's really hard to rank them against each other. I have been ranking mons within the tiers, but at this point it gets very subjective. I wouldn't say Cune is definitely better than Infernape, but this is my vague feeling atm. Both can sweep against certain weakened teams, and both can provide you a way to survive against Gyarados or TTar for example, so I think it's really hard to compare since it's mostly a question of how you think those defensive utilities compare, which depends on the meta and can change quickly.
B1 Rank
Blissey Swampert Kingdra Milotic Aerodactyl Clefable Hippowdon Flygon Abomasnow
These are upper mid tier mons in my book, which means they are often impactful on a game, but risk being a liability against certain team matchups. Aggressive, fast-paced teams can exploit stuff like Blissey and Swampert and create holes that their teammates will have to patch up. I think Aerodactyl is a good lead, but doesn't help as much mid-game since it can't threaten to explode or anything like that, so it's quite a bit lower than Azelf and Metagross for me. I think Flygon is extremely overrated a scarfer. It's fairly easily to use due to hazard resistance and sand immunity, but really quite mediocre in damage output, and it usually clicks u-turn a bunch and then has to 1v1 something in the end. I think mixed Flygon is pretty good though, so I'd keep it at least this high overall. Clefable is overrated for similar reasons to Flygon - it's easy to use because you don't need to worry about hazards or weather, but if the game is lasting long enough for you to switch in Clefable 2 or 3 times your opponent will be able to exploit it with Loom or Lucario or Infernape or Taunt Heatran or Jirachi or something else. This rank is generally where I put passive or stall teams as a whole in this meta. Kingdra is a bit of an odd one out here, but it's a solid attacker held back by usually needing to set its own rain, and not quite having enough bulk to justify it. It also doesn't have the bulk and resistances of mons in the tier above like Suicune and Zapdos and Gliscor etc in order to have the same defensive utility, so here it goes.
B2 Rank
Smeargle Jolteon Roserade Uxie Cloyster Machamp Nidoqueen Kabutops Ludicolo Qwilfish Raikou Shaymin Tentacruel
More upper mid tiers, but not as consistent as the ones above. A good player with a good team can have success with them. Smeargle, Uxie, Cloyster, and Roserade can be tricky and annoying supporters, but there are better hazard setters in the meta. This is also where I'd rank rain teams as a whole. I think they are possible to win with at a high level but are less consistent and require some predicting. Machamp and Shaymin need a bit of luck to break through, but can create a lot of problems for slower passive teams.
B3 Rank
Heracross Electivire Togekiss Quagsire Slowbro Vaporeon Donphan Weavile Hitmontop
The last of the upper mid tiers. I would say these are also the last of the respectable choices for a serious team, since they each do something unique that other mons above don't necessarily do. They are a bit of a "one trick pony" type though, and require creative teambuilding to justify. Quag/Slowbro/Vap are all some kind of Milotic alternatives that are just a bit less efficient but provide something Milotic doesn't (more immunity/twave/wish). Electivire, Heracross, and Weavile are a bit soft-hitting, and have like only one button that they can switch into, but they do create unique attacking opportunities. Hitmontop and Donphan are niche respectable spinners that provide some defensive utility, but yeah, hard to justify using them over Forre or Starmie. Everything below this is a bit questionable.
C1 Rank
Moltres Cradily Yanmega Spiritomb Registeel Snorlax Porygon2
Not really much to say for these lower tiers, since I have rarely faced or used them in recent matches, so I'm going a bit off of theory at this point. These first seven mons each have a clear use purpose, which is why I have them as high as I do, but they are also very inefficient and require some support and meta knowledge to pull off. You're not totally clowning by using them, but you are making your own life a bit harder than it needs to be.
C2 Rank
Walrein Rhyperior Charizard Staraptor Claydol Camerupt Feraligatr Honchkrow Torterra
I would say these Pokémon are almost gimmicks. I bet you could find reasons to use Feraligatr over Gyarados or Torterra over Gliscor in some circumstances, but I feel like you're going to regret not using the top or high tier mon more than 90% of the time. Stuff like Walrein and Rhyperior can get silly wins with tons of support, but I feel like you need so many things to go your way to win with mons in this rank that you probably would have won using a top tier multiple times over. Claydol and Camerupt are okay supporters with access to explosion, but their weaknesses make them very questionable to try and use.
C3 Rank
Venusaur Porygon-Z Crobat Slowking Umbreon Dusknoir Hariyama
I think these are the last of the mons I would consider having any meaningful use, unless there's some I'm forgetting, in which case, they're probably not that good anyway. Everything below this I would consider outclassed.
