Dark
These two are really good against popular types such as Dragon, Flying, and Ground in Weavile's case, and Water and Ground in Zarude's, so it felt appropriate to bump them up as they've pretty much become staples on Dark nowadays.
Dragon
Dragonite has dropped down in viability significantly over the past few months as it's very predictable, relies on setting up to be an efficient breaker and usually has to pick between Earthquake and Fire Punch unless you sacrifice Extreme Speed, and once it has revealed its move of choice it becomes much easier to deal with.
Kyurem is great at threatening Flying, Water, and Ground-type teams, all of which are super common in the current metagame.
Duraludon isn't up to par with the other Pokemon in the B rank and doesn't have any valuable niche to keep it there.
Although it's a powerful wallbreaker, Latias is far more common thanks to its access to Healing Wish which is pretty valuable, and you should never be running both.
Dracovish is an incredibly powerful wallbreaker that many teams struggle switching in to, and therefore it rises.
Toxic Spikes plus the ability to switch in on special Fairy-type attacks boosts Dragalge up a rank.
Kommo-o gained some popularity during MWP thanks to its ability to threaten Steel and Dark-type teams with its Iron Defense set combined with Body Press and Earthquake.
Electric
Magnezone is seen on nearly every Electric-type team thanks to its great typing so it didn't feel appropriate putting it in the same rank as Rotom-C and Thundurus which are pretty rare.
Dracozolt has recently gained popularity with its Substitute set catching foes off guard and being a strong breaker with good coverage.
Fairy
Clefable's the best Stealth Rock user on Fairy and has access to Knock Off which is pretty valuable especially when it comes to removing Heavy-Duty Boots. Its also able to deal with some threatening Pokemon such as Celesteela if it's holding Rocky Helmet.
Diancie doesn't quite fit in with the other C rank Pokemon as they all have more valuable roles, so it goes down to D.
Sticky Webs aren't valuable enough to keep this thing above D rank.
Primarina is completely outclassed by Tapu Fini so it's getting unranked.
Fire
Outside of setting up Screens, Rotom-H isn't all that useful despite its Electric typing, so it drops down a rank.
Victini's powerful V-Create and access to Bolt Strike makes it a great addition to Fire-type teams.
Moltres is the superior Ground immunity thanks to its better bulk, higher Special Attack, and access to two useful abilities in Pressure and Flame Body. Charizard relies on sun being up to be an effective wallbreaker and usually needs a Choice item to be good, meaning it has to forego Heavy-Duty Boots to patch up its glaring weakness to Stealth Rock.
Incineroar mainly shines against Ghost and Psychic-type teams, two types that Fire already has answers to in the form of Blacephalon, Volcarona, and Heatran which fulfill other useful roles as well meaning Incineroar drops down.
Volcanion provides a Water-type immunity which is very valuable for Fire-type teams, especially when dealing with Pokemon such as Dracovish.
Flying
Dragonite has dropped down in viability significantly over the past few months as it's very predictable, relies on setting up to be an efficient breaker and usually has to pick between Earthquake and Fire Punch unless you sacrifice Extreme Speed, and once it has revealed its move of choice it becomes much easier to deal with.
Zapdos-G is a great wallbreaker with very good coverage that many teams struggle switching in to. It especially shines against Dark and Steel-type teams, both of which are popular right now, so it goes up to A.
Grass
Zarude has a nice typing that helps Grass-type teams threaten Ghost and Psychic types, and it's definitely better than the other Pokemon in the B rank.
Appletun has fallen off from Grass teams ever since the return of Cradily, so it drops down another rank.
Ground
Garchomp doesn't belong in the same rank as the likes of Gastrodon, Nidoking, and Landorus-T, all of which are much more useful than it and give a lot more to the type.
Despite it's high Speed and access to Extreme Speed, Zygarde-10% is very weak even when equipped with a Choice Band, and its signature move Thousand Arrows isn't enough to keep it from dropping a rank.
Ice
Piloswine is better than Mamoswine as a Stealth Rock setter because of its bulk which covers Ice's lack of defensive utility. Mamoswine is still usable but harder to fit.
Cloyster is better than Avalugg for hazard removal, because it can set hazards which means it isn't completely blocked by Ghost Pokemon at the same time.
Kyurem beats a lot of top types almost single handedly. Substitute sets are incredibly dangerous and provide Ice teams a wincon in many different matchups.
Weavile fills an important niche as Ice's best Speed control and a dangerous offensive threat. A good Knock Off user is also notable for weakening Celesteela and removing Heavy Duty Boots.
Dangerous wincon in matchups like Poison, Fairy, Normal, Flying and Psychic; if it's not outright winning its natural bulk is nice to serve as a natural check to certain threats.
Both these Pokemon suck so they are being unranked.
Psychic
Slowbro's an amazing pivot for Psychic-type teams thanks to its good bulk paired with its Regenerator ability and access to Teleport. Its good Defense and solid typing also allows it to check many physical attackers.
Bronzong provides a solid Ground immunity to check the likes of Landorus-T and Sand Rush Excadrill, and has access to Body Press which is nice against Dark-type teams.
Steel
Skarmory's ability to check Urshifu-R and other physical attackers with its incredible Defense and Rocky Helmet makes it rise a rank.
With Celesteela and Skarmory both being so good right now, Corviknight doesn't see much usage and drops down a rank.
Water
The rise of Skarmory poses trouble for Urshifu-R as it can live its attacks comfortably and whittle it down with Rocky Helmet.
As mentioned above, the rise of Skarmory now gives Keldeo an edge as it's better than Urshifu-R at breaking Steel teams that have Skarmory.
Crawdaunt's a very powerful wallbreaker and is immune to Psychic-type attacks which lands it in the B rank.
A -> B 
(Water) A -> B
(Water) C -> A 
B -> A
B -> C
(Water) C -> B 
B-> D (Grass).
C-> B (Grass).
B-> C (Grass).
D-> B (Electric).
A-> B. 
B-> A (Ice). 
S-> A.
D-> B 
A->B. (ICE)
