After making the SWSH sets compendium, I have a lot better understanding of the meta. There's a few Pokemon that I'm either constantly or never preparing for and I think that's a very important factor in viability. I also know what relatively untested lower tier mons are capable of now. Here's a long chain of justifications:
A -> S-
Sylveon is an incredibly powerful Pokemon in the current meta that can play as a brutal breaker, potent stallbreaker, or customizable Yawn user. The fear of Hyper Beam or Yawn forces so many to run Sub which is easily exploited by Hyper Voice and the prevalence of our 600 BST Dragons makes it a very good check to them, especially when running its physically defensive set. Besides the few Band users who can brute through it, almost every Pokemon either loses to or is forced to prepare especially for Sylveon making it extremely influential and splashable.
A- -> A
Crustle is a benchmark I was constantly prepping for in sets. It's another Pokemon of huge influence and depending on its item or plays, it can beat tons of Pokemon that would like to beat it such as getting through Taunt Mew with Mental Herb, outspeeding a Dragapult with Endure+Salac, and cheesing out KOs with Hard Stone.
A- -> B
Whimsicott has fallen off and relies on its berries and last slot to beat the things it wants to. The prevalence of Poison coverage from random mons like Mew and Sawk, multi-hit from Crustle and Rhyperior, and sound from Sylveon give this thing a brutally hard time and meanwhile Whimsicott is struggling to beat Grass-types and either Dark-types or anything with sustain.
B+ -> A
Hydreigon is blessed with a speed tier of 98, outspeeding Haxorus by 1, and SpA for 125, giving it the hardest hitting Draco around. Most Pokemon simply can't afford the EVs to tank a Modest Specs Draco from Hydreigon and it's also the best Haban user in the tier imo being capable of beating every other Dragon type but Band Dragapult, Scarf Unnerve Haxorus, and Specs Goodra. It also packs some utility with Taunt, Nasty Plot, and Snarl, can make mons like Tyranitar drop dead to Focus Blast, or run Scarf. Hydreigon is massively undervalued right now.
B+ -> A-
From how I've spoken about this mon, Mega Gengar, and Jirachi last gen, you should know very well how much I dislike these kinds of strategies but the power of Togekiss is undeniable right now. Flinches aside, Togekiss has access to Trick (for Scarf) and Encore (for Life Orb), giving it great matchups against stall. Its high stats and Fairy typing let it take on all of the Dragon spam going on but it's not completely helpless against Steel types thanks to Fire Blast. And yes, Air Slash; Togekiss can and will win matchups it has no business winning with this move making it a very splashable pick.
B+ -> A-
Expert Belt Tyranitar is an insanely anti-meta Pokemon capable of taking on 5/7 of the highest ranked Pokemon and maintaining reasonable matchups throughout the rest of the VR thanks to its raw stats, STAB speed control, and Taunt access. There's not much to say about this mon other than the fact that it does work.
B -> B+
Arcanine is often seen from an offensive perspective and while Band and Life Orb are
good, this Pokemon thrives as a defensive tank thanks to Intimidate, Will-O-Wisp, Snarl, great stats, and even the ability to void its typing when necessary with Burn Up. Another Pokemon that simply does work.
B -> B-
Duraludon as an incredibly awkward Pokemon to use in my experience. Its speed isn't bad but falls just below the point of usefulness, its SpD is trash to the point that it needs Assault Vest to check opposing Dragons despite its amazing typing. It has an incredibly shallow movepool and while tech like Rock Tomb and Metal Burst sound cool on paper, they fall victim to Substitute as do Draco Meteor and the less good Steel Beam.
B -> B-
Rhyperior isn't a bad Pokemon on paper but simply faces great competition from Crustle and Tyranitar. While there's a few Pokemon it can take care of like Excadrill which they can't, it's abysmally slow and forced to invest extremely heavily in defenses.
B -> B+
Band Snorlax is a bulky Pokemon with awesome coverage. It can fall victim to certain techs and doesn't do well against things it can't hit super-effectively but it nonetheless does a great job taking down FDS cores with Heavy Slam, Outrage, and Heat Crash.
B- -> B+
Aromatisse works in strange ways but gets the job done nonetheless. It threatens every Choice user with a sneaky Disable and easily beats up on Pokemon that can't 2 shot it Encore+Disable. Aroma Veil is an awesome boon against Pokemon like Corviknight, Mandibuzz, and non-Mental Herb Mew and I think that we can expect this Pokemon to see an increase in usage in the future as people better learn its ins and outs.
B- -> B
Grimmsnarl's Bulk Up set functions as a blanket check to physical attacks beating Band Dracovish and having a chance vs Band Darmanitan while having excellent stallbreaking capabilities in the form of Prankster Taunt and not autolosing to special attackers. I was bordering on B+ for this but I think we'll see where things go in the meantime.
B-/C+ -> B
Both of the main Rotoms perform excellently as defensive Pokemon and Choice Scarf users. Their Ground immunity and unique typings give them distinctive niches and they can pack a very surprising amount of utility. They're not without issues but can be very difficult to play against and are definitely under-prepared for.
C+ -> B
Rillaboom is imo the best Leech Seed user in the tier, at least until Mew is banned. Unlike Ferrothorn and Eldegoss it has reliable Mimikyu and Aromatisse matchups and a beats every Imprison Mew but Taunt+Sitrus+Substitute. While you'd think it has no business taking on Golisopod and Weezing, it can and will beat them and if you're willing to drop Protect and its matchups, you can abuse Drum Beating+Taunt to shut down any non-Grass stall Pokemon.
C -> C-
As someone who's actually used it, Silvally is not a good Pokemon. The only good thing about this Pokemon is set ambiguity and besides taking top-tier teambuilding to take advantage of, the schrodinger's Silvally tactic is unreliable at the best of times and hugely matchup-dependent. Borderline D rank for just how gimmicky it is.
C -> B-
Copperajah hits like a truck and is a great blanket check to FDS much like Snorlax. It ironically has some issues that arise from the type matchups of being a Steel-type and needs to invest too much in defenses like Rhyperior but still manages to be anti-meta and can even edge around Haban by opting for Sheer Force Play Rough instead of Outrage.
C+ -> B-
Steelix is very similar to Copperajah in theory and in practice. While it loses out on some raw power and great coverage options, Sturdy eliminates any rolls vs Fire-type attacks and its STAB EQ gives it a better matchup vs certain Steel-types.
C -> C-
Meh. That's all there is to Gardevoir. It has okay coverage, an okay typing, okay damage output, okay speed, okay utility... need I go on? It has a hard time doing much as a physically-frail Pokemon in a physically-oriented meta dominated by bulky offense.
C -> UR, C -> D
Indeedee is a really bad Pokemon, especially its male form. It doesn't get half the kills you'd expect, its speed tier is irrelevant, and it requires way too much bulk to survive anything. Male is 100% outclassed by female and female is very close to being outclassed by Togekiss, having only 1 matchup over it and beating about 10 Pokemon total.
C -> C-
Obstagooon has decent bulk, decent damage, decent utility, and decent speed, putting it a similar but slightly better position than Gardevoir. What holds it back however is its rigidity. This Pokemon would kill for some move options like Sucker Punch, Rock Tomb, or even Quick Attack.
C -> D
Psychic just isn't a good typing. Prevalent Pokemon Mimikyu, Dragapult, Hydreigon, and Golisopod immediately shut Reuniclus down and contrary to what its stats might suggest, Reuniclus doesn't get much use out of Calm Mind. Acid Armor isn't the worst but there's better physical walls out there.
C -> D
Umbreon is pretty heavily outclassed by Mandibuzz but finds a niche in its pure Dark typing, allowing it to beat just a couple Pokemon like Darmanitan and lose several others ignoring the issue of its speed tier. This should be reflected in its ranking as using it has a huge opportunity cost.
C -> C+
Vaporeon is pretty neat. Its neutral typing, great bulk, and access to Yawn let Vaporeon beat most of the tiers bulky offense blow for blow and even crack a couple stall Pokemon. Has some issues with Substitute users and Yawn RNG but not as outclassed by Sylveon as people tend to say it.
C -> B-
Wishiwashi is the real Pokemon that Vaporeon sees competition from. Wishiwashi has outstanding bulk and damage output and while it's utterly inflexible and loses a few matchups that Yawn grants, its raw damage and superior reliability make it a great tank.
C- -> C+
Appletun is a really cool Pokemon that deserves better. Ripen+Sitrus+Iron Defense/Amnesia allows it to wall almost any Pokemon that can't OHKO it and Apple Acid provides great stallbreaking capabilities. Haban is also very cool and it manages to differentiate itself from other Dragons with access to Sucker Punch and Recover.
C- -> D
Eldegoss can do a couple things that Rillaboom can't like beat Band Haxorus/Sawk thanks to Cotton Guard but is otherwise outclassed. Very similarly to Umbreon, while it's not a bad Pokemon, it faces huge opportunity cost.
D -> C+
In addition to a slightly gimmicky stall set, Centriskorch is a really effective Choice Band user capable of beating Mew, Darm, Dragapult, Corsola, Sylveon, and more. Check it out!
D -> C
Tsareena may seem outclassed by most other defensive Grass-types but the combination of Taunt and Rapid Spin allows it to break Pokemon like Corviknight and Mandibuzz giving it a unique niche. It also has very surprising natural bulk.
C-/D -> UR
Quagsire is outclassed by just about every other bulky water in the tier, Sirfetch'd is outclassed by Conkeldurr, and Orbeetle and Trevenant are just plain bad. Let me know if you find a reason to use any of these mons.