I'll go ahead and explain the rankings a little bit:
S Rank:
Corviknight is the best Defog user in the tier and having that role while also having superb bulk, excellent typing and access to great moves including Taunt and Roost puts it at the top of the viability rankings. It is extremely easy to slap it onto teams as a Defog user that can check/counter a good portion of the tier, including Ground-types like Excadrill, Steel-types such as opposing Corviknight, and Fairy-types such as Hatterene. It can also be used as a pivot into other threats such as Dragapult or Clefable thanks to its bulk. Taunt allows it to shut down slower and more passive mons like Ferrothorn, usually to prevent hazards from being set up or to prevent slower mons from setting up. It can usually choose to run Iron Head / Body Press / U-turn / Brave Bird, and/or it can choose to run Bulk Up to turn into a bulky sweeper. Lastly, Pressure complements Corviknight quite well as it can quickly PP stall moves such as Mystical Fire from Fairy-types, and Hex from Dragapult. Pressure with Taunt can usually create a situation where the opponent is forced to attack and this usually results in them getting PP stalled more quickly, or them switching out. Overall, there is a tremendous amount of upside to using Corviknight and nearly no downside to using it.
A+ Rank
Clefable with Life Orb is an incredible breaker, which has resulted in the metagame adapting to it with new mons/sets such as Sylveon, Flareon, and specially defensive Clefable. The specially defensive set provides great utility with Stealth Rock, Wish, and bulk to handle special threats such as Dragapult and opposing Clefable. Magic Guard is fantastic as it allows Clefable to safely sit in front of mons like Seismitoad that can pressure opponents thanks to status.
Dragapult is the fastest mon in the tier and can pressure opponents thanks to it's coverage, decent power, and variety of sets. Ghost-typing is incredible this gen, especially with Pursuit no longer being available, and Dragapult has the tools to punish common switch ins such as Mandibuzz with Thunderbolt or Will-o-Wisp. Thanks to its speed, it is able to force out offensive mons regularly and immediately pressure teams or create an opportunity to switch to a teammate. Infiltrator also allows Dragapult to prevent Substitute users, such as Hydreigon, from staying safe while attempting to sweep.
Mandibuzz is the next best Defog user in the tier and it is one of the few Ghost resists in the tier. Mandibuzz is able to somewhat reliably switch into Dragapult and other specially attacking threats like Hydreigon, and then remove hazards or remove opposing items with Knock Off. Knock Off is fantastic this generation thanks to no mega pokemon nor any z-crystals being available, and this allows Mandibuzz to apply a little bit of pressure. Alternatively, it can grab momentum after forcing out Dragapult/Hydreigon with U-turn and stay healthy thanks to Roost with Heavy-Duty Boots.
Rotom-Heat is a big threat to a lot of teams thanks to its new move in Nasty Plot alongside its coverage and power in Volt Switch/Discharge + Overheat. While Seismitoad may commonly switch in, having to take at least 40-50% from a +2 Overheat can cause it to get overwhelmed in the long run unless it has Wish support. It is able to switch in against many Fairy and Steel-types, including the highly common Clefable and Corviknight, and set up the following turn. Darmanitan and Dracovish forced users to run bulky water types, usually Seismitoad, Jellicent, Milotic, Ferrothorn, Toxapex and Gastorond, most of which Rotom-Wash is able to set up through forcing out or flat out KO.
Seismitoad is a fantastic Stealth Rock user thanks to its defensive typing alongside its ability Water Absorb, which allows it to reliably check Dracovish, Dracozolt and Rotom-Heat. Seismitoad can also pressure switch ins thanks to status conditions in the form of Scald burns and Toxic. Providing a defensive answer for several big threats while simultaneously providing Stealth Rock and being capable of applying a little bit of pressure thanks to Status has caused Seismitoad to see high usage and land a spot in the A+ rank.
Toxapex is a great physically defensive pivot that can also spread status via Scald, Toxic, Toxic Spikes and Baneful Bunker. The common core of bulky water / Corviknight or Mandibuzz / specially defensive mon, usually being Seismitoad + Corviknight + Clefable, is something that can find Toxapex frustrating not only because they can't KO Pex, but because they will have to send in something to KO Toxapex that doesn't want to be statused, such as Hydreigon. During that time, Toxapex may set up Toxic Spikes, and because it has a good matchup against the common hazard removal (Corviknight, Mandibuzz and Excadrill), it can give them a hard time when trying to remove Toxic Spikes as Toxapex can punish with Scald or Toxic as well. Finally, Haze allows it to continue staying alive and prevent sweeps from threats like SD Aegislash, Bulk Up Corviknight and Curse Snorlax.
Overall, the mons in S and A+ are extremely common and provide extremely great value with little cost.
A Rank
Aegislash is a very big threat thanks to Ghost-typing being an excellent offensive typing and its ability to go physical, special, or even mixed. It has a lot of sets, including Choice Band, Choice Specs, Swords Dance, Metal Sound and whatever you want really. Switching into it can be difficult due to having to accurately guess between the above sets, namely the first two as a specially defensive mon such as Mandibuzz will fall victim to Choice Band sets, while a physically defensive pivot such as Seismitoad will take a tremendous amount of damage from Specs Aegislash. While it boasts great power, it may not flat out 1HKO much, but it can 2HKO nearly everything and if it is able to attack while forcing a switch, then it is truly a big threat.
Despite its look, Dracovish is actually the hardest hitting pokemon in the tier thanks to Fishious Rend along with Strong Jaw. Dracovish is very centralizing as teams are somewhat forced to run either a water absorb mon, or 2 sturdy water resists in order to sufficiently switch into it. Similar to Darmanitan, it can still deal very solid damage with Fishious Rend while holding a Choice Scarf, which makes it difficult for offensive pokemon outside of Timid Dragapult to revenge kill it or force it out. Choice Band sets trade the ability to outspeed most of the tier for additional power to break past normal checks like Seismitoad (2HKO'd by Outrage), Toxapex (2HKO'd by Psychic Fangs), and Ferrothorn (2HKO'd by Low Kick). Either way, it can be a headache to switch into at times and is fairly reliable mon.
Dugtrio needs little explanation as it is still pretty good at trapping grounded threats with Arena Trap. It can trap and KO weakened threats like Clefable and Hatterene, or it can use its Focus Sash to trap faster threats and priority users, such as Excadrill after Rapid Spin and Sucker Punch Bisharp. While its ability to trap remains the same, it does struggle a bit more than it did in the previous generation due to losing Tectonic Rage, and also having less targets that can be 1HKO'd.
Excadrill is the most reliable Rapid Spin user in the tier and can pose a large threat thanks to Rapid Spin seeing an incredible buff, and Mold Breaker ignoring Levitate on common mons such as Rotom-Heat and Hydreigon. This means that Flying-types are the only safe immunities to Earthquake, and while this is great, Flying-types still have to deal with Iron Head, which something like Mandibuzz won't appreciate. Corviknight on the other hand can safely switch into Excadrill and proceed to Taunt it to prevent setup, or use Body Press to KO it. Corviknight's presence in the tier plays a part in Excadrill from being in a higher rank.
Ferrothorn was able to keep its most useful moves, including Knock Off and Toxic, both of which have fewer users this generation. Ferrothorn is the best Spikes user in the tier and is able to easily switch in on passive mons like Seismitoad and then proceed to set up hazards, or Knock Off an opposing item. Additionally, because Ferrothorn is the only Grass-type in the tier, Leech Seed is able to pressure everything outside of opposing Ferrothorn, and Magic Guard/Bounce users in Clefable, Hatterena and Sigilyph. However, Ferrothorn does have to deal with most Fairy-types carrying Fire coverage, which prevents it from switching in as easily as it wants to. Ferrothorn can also switch into Dracovish's Fishious Rend and proceed to threaten it with Leech Seed/Toxic + Iron barbs/Protect.
Hydreigon is a big threat to balance teams thanks to its new move in Nasty Plot this gen. Substitute sets are able to safely get a sub up against passive mons like Seismitoad and Toxapex, and then get a Nasty Plot off somewhat easily. Flash Cannon being able to hit Fairy-type makes it difficult to switch into, which results in Mandibuzz being the most common switch into Hydreigon. Luckily, the metagame has adapted a little bit and specially defensive Clefable can tank 2 Flash Cannons to force it out, while Sylveon can tank +2 Flash Cannon and immediately KO Hydreigon even with a sub up. However, while Sylveon and Clefable may be hard for Hydreigon to immediately break, Draco Meteor is now an option to potentially 1HKO Mandibuzz, which is not only a common switch in, but is also fairly likely to be the opposing team's only Ghost and/or Dark resist.
Every mon in the A rank can pose a big threat to any team, but they may possess a few extra hurdles to cross in order to maximize their value when compared to the mons in the A+ rank.
A- Rank
Conkeldurr is a fearsome wallbreaker that is pretty tough to switch into. Guts allows it to ignore status conditions from mons like Seismitoad and Toxapex, and then grants Conkeldurr respectable raw power to 1-2HKO a majority of the tier. Drain Punch keeps it healthy and makes it difficult to wear down, Earthquake and Thunderpunch provide great coverage, and finally, Mach Punch allows it to threaten faster offensive mons that may try to revenge kill it like Dugtrio. Conkeldurr's main downside would be its low base speed, which causes it to be outsped by Life Orb Clefable, Aegislash, both of which take little to nothing from Mach Punch.
With dynamax banned, Ditto has seen a sharp decline in usage and it now sits in A-. There are far less set up sweepers nowadays and Ditto is unable to be as valuable as it once was. It can still provide great utility through speed control/revenge killing opposing threats such as Nasty Plot Rotom-Heat and Dugtrio (you can trap them back and outspeed to KO them). As long as Ditto is capable of punishing offensive mons (which it always will), then it will continue to remain viable.
Hatterene, while incredibly slow, can turn into a dangerous sweeper thanks to Calm Mind, coverage options in Mystical Fire, and Magic Bounce, which prevents it from being phased out by Whirlwind, or directly statused by Toxic. Draining Kiss can act as somewhat reliable recovery at times, and Psychic alongside Fairy and Fire coverage can be difficult to safely switch into. Hatterene does have to deal with Corviknight, which can tank a +1 Mystical Fire and Iron Head back, and then faster threats which can force it out with ease. Due to its low speed, the number of faster threats refers to a good size of the tier as base 29 speed is extremely low.
Hippowdon is a solid mixed wall that can check threats like Excadrill, Aegislash and Dracozolt, while simultaneously providing Stealth Rock. Slack Off is great recovery to help Hippowdon remain healthy in the long run, and it can also phase out set up sweepers like Calm Mind Reuniclus and Bulk Up Corviknight. Hippowdon's main downsides are the slow speed stat, along with being threatened by Water-types and all 3 Rotom formes. It can also get shut down by Corviknight's Taunt and let in other hazard setters, such as Seismitoad, Clefable and Ferrothorn.
Rotom-Wash is a decent Choice Scarf user who can be difficult to handle at first thanks to Trick and Thunder Wave. Trick can cripple a switch in like Seismitoad or Ferrothorn, and Thunder Wave can paralyze fast threats like Dragapult, whom you can outspeed thanks to the Choice Scarf. Being immune to Dugtrio and Spikes is nice, and Rotom can also provide momentum via Volt Switch to get teammates in to wallbreak or setup. The downside to Rotom-Wash is that it struggles offensively and mainly needs to rely on Thunder Wave and Volt Switch to apply pressure. While that doesn't sound too bad, it can taken advantage of fairly easily by and forced out, especially after Tricking away its Choice Scarf.
Despite losing Pursuit, Tyranitar continues to be a good choice in OU. Its best set is the Choice Band set, which can be difficult to switch into as Crunch + Stone Edge can hit most of the tier very hard and weaken physically defensive pivots like Seismitoad. Defensively it can switch into Dragapult and Rotom-Heat, although it does have to worry about being burned by Will-O-Wisp. Tyranitar also runs a Stealth Rock set and is a decent setter thanks to its ability to pressure Corviknight, Mandibuzz and Excadrill with Stone Edge and Fire Punch. The downsides to Tyranitar include its speed tier, which can be nice at times due to it being faster than Clefable and Aegislash, but it is still fairly slow and is outsped by Dracovish and Dracozolt. Tyranitar is also trapped by Dugtrio, which means that you can lose it fairly easily even after getting a KO. Finally, Tyranitar dislikes bulky water types, which are very common at this point in time due to teams wanting to be prepared for Dracovish.
This is a bit of a repeating pattern, but the mons in A- carry a heavier cost or more flaws with them than the mons in the ranks above. These mons can be threats, but are also less splashable than the mons above them.
I'll end this post for now and do the mons in B rank next time and C ranks in the post after that. You all are more than welcome to ask questions or correct me on anything I posted above. Happy new year and here's to the first year of Generation 8 and the first day of the new decade. May you all have success with everything you do, reach new heights beyond your limits, and live life to the happiest while developing in whatever ways you choose.
Explanations for B Rank mons (post #78) can be found
here.
Explanations for C Rank mons (post #81) can be found
here.