Gonna post my VR here, take it with a grain of salt as my peak is only #96, so im not very good, just passionate about SM and wanted to share my thoughts.
S Rank

Magearna

Landorus-Therian (Self-explanatory, near-guaranteed fit on any offense team and always fills in the gaps in a team. Not as versatile as Mag however, and it doesnt quite fit on EVERY composition)
S- Rank

Toxapex

Tapu-Lele (Specs variants are arguably the strongest wallbreaker in the tier, doing considerable damage to even the tankiest of resists. Other variants like Choice Scarf, Taunt + Natures Madness and Z-move CM also fit well on offense, and entire teams can be built around psychic terrain)
A+ Rank

Mega-Alakazam (Being the fastest mon in the tier, its hard to find a game where zam didnt contribute anything at all)

Kartana (Amazing mon, personally dont think its as broken as its made out to be, but still very powerful. Its also surprisingly versatile, as recent experiments with Band/Utility kart on balance teams have panned out well)

Gliscor

Tornadus-Therian

Mega-Latias

Mega-Medicham (Anything with 100 base speed shouldnt be hitting this hard. It can absolutely rip through any team that doesnt have a reliable check to it; Cresselia, Meta Latias, Reuniclus, Mega Sableye, Slowbro, etc)

Heatran

Mega-Charizard-X (Not only is an amazingly powerful sweeper than can grab random OHKOs with Flare Blitz, but has a great deal of value as an SD wallbreaker on slower paced teams)

Chansey (Cornerstone of balance and stall, the tier would fall apart without chansey)

Serperior

Tapu-Koko (Its hard for me to understand the negative attitude towards koko in the modern meta. It has a variety of great sets, and the natural speed control it brings + its versatility allows it to fit on almost any archetype)

Ferrothorn

Ash-Greninja (Although its an incredible wallbreaker, its strength has invited many common and niche checks to it into the tier. You'd be hard pressed to find a good team without at least 1-2 ash-gren checks)

Mega-Charizard-Y (Unorthodox pick here, but this mon has the least switch-ins out of anything in the tier (like just M-latias and chansey lol). As I said with medicham, it has a great speed tier for how hard it hits, and Fire/Flying + Roost + Good Natural Bulk grants it more longevity and ability to switch into attacks than most wallbreakers, even despite its major SR weakness)

Greninja (Definition of versatile, scouting for its set can be difficult because of the sheer amount of Z-moves and coverage options it has. It doesnt hit as hard as other offensive threats, but a bad prediction against it can lose the game instantly)

Kyurem-Black
A Rank

Mega-Mawile (Can perform its wallbreaker role in so many ways, and with so many coverage options + SD, it can fit like a glove on anything ranging from offense to balance)

Weavile (The offensive utility and power weavile can bring to a team is hefty, with it having STAB Knock, Pursuit and STAB priority with Ice Shard. Works well on literally any archetype, whether that be as a supportive pursuit trapper/knocker for mons like gliscor, or as an offensive check to ice-weak pokemon like Garchomp, Tornadus-T, Gliscor and Hawlucha)

Clefable

Volcarona (Arguably the most versatile sweeper, volc has a lot of prescence in the tier and any team without a check to it will surely be blown away. Offensive sets work well on any HO or offense team with good hazard control, while bulky sets serve as a great check and punish (with Flame Body) to common mons like Kartana, Excadrill and Weavile for balance teams, and its even great on stall as a cleaner)

Cresselia (One of those choose-your-own-checks mons, with crazy enough stats that allow it to mix and match its EVs to fit almost any balance/BO composition. Its typing guarantees it works as a check to the plethora of psychic and fighting types in the tier, even without any investment. Many teams can just auto-lose to CM cress too)

Hawlucha

Rotom-Wash (One of the few mons that is equally as valuable against offense as it is against stall/balance. Great roll compression and works on most team archetypes)

Mega-Lopunny

Excadrill

Mega Diancie (Great speed tier and hits very hard, while also having great utility in magic bounce and amazing stab moves and coverage. Unfortunately, its typing combined with its average bulk and lack of ANY recovery (cant even access lefties) leads it to be worn down very easily, and it not having any boosting moves hinders its ability to break through walls as well)
A- Rank

Mega-Tyranitar (Despite it being a wall of stats, its typing leads to it being threatened out by a great deal of things, and its lack of recovery or leftovers combined with its vulnerability to every type of hazard makes it very easy to wear down. It is undeniably very strong and adds great value by virtue of SR and hitting things with an 164 attack stat)

Tapu-Fini (One of the best BO mons in the game. Works as an amazing check to all the threatening water, fighting and dark types, and has great utility with Defog + Taunt, as well as its trapper set)

Manaphy (Cheesiest mon in the tier, Rest + Hydration ruins most defensive counterplay against it, and 100 stats across the board allows it to take super effective hits and just full heal them off. Higher than pelipper and swampert because its not a rain-exclusive, buts its best value comes from rain teams)

Pelipper +

Mega-Swampert (Rain is such an easy archetype to use. It either completely destroys teams that dont have a sturdy check to it, or get rolled by teams with multiple water + ground resists)

Kommo-o (The standard z-sweeper set can feel underwhelming, however the SpDef set is an amazing check to many common mons like ash-gren, volc, heatran and serperior. DD Z-thunder punch sets are great to catch people off guard who think Celesteela or Toxapex are checks)

Mega-Scizor (Bulky set is great at sitting on things, and it can customize itself with a great movepool, acting as a U-turn pivot, or a tanky defogger. The fully offensive SD is extremely underrated and can completely sweep teams that dont have psychic terrain, or a hard check like skarmory, bulkarona or zapdos)

Tapu-Bulu

Mega-Pinsir (Hits extremely hard for how good its speed tier is, and having a strong flying type priority comes up very often for dealing with fast offensive threats, like greninja and alakazam, while also revenging the numerous flying-weak sweepers like volcarona, kommo-o, serperior and hawlucha. Fighting/ground coverage means it doesnt auto lose to every steel type in existance, and pairing this with magnezone can remove the few checks that do exist for this)

Garchomp
B+ Rank

Ditto

Victini (Powerful scarfer that can revenge kill many things by virtue of its bulk and an incredible "signature", v-create. Unfortunate, its horrendous typing leaves it vulnerable to SR which is not ideal for a pivot. Often times it gets the most value by clicking Final Gambit turn 1)

Celesteela

Magnezone (One of the best enablers ever conceived, trapping steel types can benefit so many mons, both offensive and defensive. Pairing it with things like Kartana, Mega Pinsir, Mega Scizor, Serperior, Tapu Lele, Kyurem-Black, Tapu Bulu, Tornadus-Therian, Scolipede, Clefable, Gliscor and so many others will allow them to do their job much more effectively, without having to worry about getting walled and/or letting steels in for free progess. Despite the immense catalog of magnezone benefactors, trapping is kinda of its only niche, and beyond that it can only act as a slow but decent special wallbreaker, in which case it is severely outclassed)

Zapdos

Mega-Gyarados (Quite a potent sweeper against more slow pace teams, as the combination of mold breaker and substitute + dragon dance can quickly overwhelm any weakened team. Not the best against offense, but its typing both in and out of mega forme combined with its excellent 95/109/130 bulk gives it decent tanking abilities)

Mega-Latios

Slowbro (Amazing check to most physical mons in the tier, like Mega Medicham, Gliscor and Mega Charizard X, and regenerator improves its matchup greatly into stall. Its typing also allows it to soft check special attackers like Tapu Lele and Manaphy)

Skarmory

Muk Alola (This is definitely much higher than many people would place this, but I think it is the most underrated special wall in the tier. Its the best switchin on Specs lele, Mega Alakazam, Mega Latias, Cresselia and Reuniclus, even better than chansey due to being able to pursuit trap for great chip damage and a 30% chance to poison. On top of this, its poison typing means it can act as counterplay to t-spikes toxapex on offense teams. It is fairly easy to fit on any team that can sufficiently cover its weaknesses to both ground and hazard chip, which isnt very difficult to achieve)
B Rank

Gastrodon (Better than seismitoad imo due to its longevity with recover, and more streamlined stats for dealing with special attackers. They are difficult to compare, as gastrodon is generally sat on more balanced teams, while seismitoad is stronger on BO)

Seismitoad (While it has no reliable recovery and worse stats than gastrodon, it makes up for that in a stronger utility movepool. SR and Knock Off are great options that provide much more pressure than gastrodon can put out, and while it needs more support with something like a wish chansey or rest + heal bell magearna, it is more suited for BO teams because of this utility)

Jirachi

Dragonite

Reuniclus (One of the biggest MU in the game, as it can either auto-win against a team not prepared for it, or provides minimal value if even one check or counter is present on the team. Despite being the so-called balance breaker, it can lose to common balance mons like toxapex (if its running 8 speed EVs, you can outrun and haze it), mega latias, unaware clefable and slowbro. It is also hard walled by mega sableye, making it a liability against stall if the sableye has not already been eliminated)

Buzzwole (A very underrated pick as a physical wall, but it is a great check to many common sweepers in the metagame. Its typing, combined with absurd 107/139 physical bulk lets it work tremendously as a stoppage to most physical attackers/boosters, including but not limited to: Kartana, Mega Scizor, Garchomp, Mega Tyranitar, Mega Gyarados, Excadrill, Gliscor, Tapu Bulu and more. Its low special bulk is a very targetable weak point, but pairing it with special blanket checks like AV magearna or chansey will wall most of the tier)

Keldeo

Tyranitar

Mega-Sableye (Stall isnt that horrible of an archetype as many would suggest, it just requires innovation to deal with a combination of the common stallbreakers in the tier, like Kartana, Heatran and SD Garchomp. Mega sableye is almost required on any viable stall team due to its phenomenal defensive utility, although it is limited to exclusively this archetype)

Mega-Garchomp (Mixed sets actually provide quite a bit of value against all team archetypes, and the amount of options it can run can sometimes make it worth it to use over normal garchomp)

Volcanion (With the correct support, this thing becomes an amazing balance breaker. Offensive sets on this are terrible thought, only use the bulky sets, whether that be AV or physical roar with leftovers + protect)

Kyurem (Great at completely PP stalling out anything slower than it, and some teams cant do anything about it. With lunar dance support to restore its own PP, this thing becomes maybe the biggest troll in the tier next to gliscor)

Hippowdon (Underrated as a sand setter, as it has more longevity than ttar because of slack off. Whirlwind, SR, yawn and toxic are all great utility options it has access to as well, so while best used on sand teams, its splashable on certain BO teams as well)

Hydreigon
B- Rank

Mega-Venusaur (Its a great stop-gap to much of the tier, and because of its insane bulk with only 2 weaknesses, it very often walls 5/6 of the enemy mons)

Hoopa-Unbound

Bisharp (Very strong cleaner due to STAB knock off and sucker punch coming off of 130 base attack. STAB iron head is great coverage for murking clefable, ttar and even a weakened magearna, and its +2 attacks even hit resists hard. It is also an auto-include on niche sticky web teams, not only because of its speed tier greatly benefitting from having webs up, but also because anyone daring enough to defog on bisharp will be giving it a +2 boost without any setup necessary. Its frailty, however, leaves it extremely vulnerable to even neutral special attackers, and checks often need to be severely weakened or dead for it to even attempt a sweep. It is a big one-trick-pony)

Thundurus-Therian

Crawdaunt

Mega-Slowbro (Even just holding the Slowbronite is helpful against reducing the damage that knock off would do, but having the ability to mega evolve in a pinch can allow it to live things it really shouldnt, like CB kartana leaf blade, CB bulu horn leech or +2 Z-Outrage from garchomp. Losing regenerator sucks, and often your mega slot is taken up by something other than slowbro, but if there is nothing available, then just having the option its very useful. CM wincon sets pair well with its crit-me-not ability, and 130 special attack on a defensive mon is nothing to scoff at either)

Gyarados

Nidoking (Nidoking can dismantle many teams if given the opportunity to switch in. Since its prediction reliant, there is some randomness when using it, but getting predicts right will net an OHKO almost every time. Very fun mon to use)

Amoonguss

Azumarill (Not as bad of a sweeper as big poké would have you believe. +6 aqua jet still hurts, and still kills most weakened things and its good bulk will allow a setup turn if youre good enough to position it properly. Its just not very desirable or splashable)

Mega Heracross
C+ Rank

Blacephalon

Mega-Aggron

Mew (Very interesting HO lead, has decent utility to enable other offensive teammates, but it isnt that difficult to defeat and its impact is often minimal. It is also very telegraphed as a lead, so being anti-led is common)

Scolipede

Klefki (While it does have its shortcomings, mainly in its stats, it has an array of amazing utility thats further boosted by prankster. Its a great role compressor, having access to dual screens, t-wave, spikes, defog, and switcheroo, and can fill the final teamslot on teams that require multiple of these things at once)

Mega-Altaria (A great typing a great defensive stats let it check a lot of things in the tier, and with pixilate+return it isnt a slouch for damage either. It also has good utility like heal bell, roar, defog and haze, but it isnt a very splashable pokemon due to taking the mega slot)

Tangrowth

Mega-Camerupt (Great switch in to all volt switch users not named rotom-wash, and hits really, really hard)

Mega-Aerodactyl

Cofagrigus (Good switch-in to the physical attackers that need their ability, like mega medicham, gliscor, azumarill, hawlucha and more. Also breaks toxapex's monopoly on toxic spikes, although it is a much worse user of it due to its vulnerability to status and chip)

Suicune

Mega-Gallade

Mantine

Mega-Sharpedo (A good cleaner because of its speed and strong jaw boosted coverage, however its also a one trick pony)

Gengar
C Rank

Breloom

Mamoswine

Porygon-Z (Z-conversion sets work, but a nasty plot + z-hyper beam can one shot chansey at +2 so that set is automatically better.
+2 252+ SpA Adaptability Porygon-Z Breakneck Blitz (200 BP) vs. 252 HP / 4 SpD Eviolite Chansey: 626-738 (88.9 - 104.8%) -- guaranteed OHKO after Stealth Rock)

Nihilego

Quagsire

Araquanid (This is the best sticky web setter in the tier, ribombee propoganda will not win)

Marowak-Alola

Mega-Blastoise (Certainly a strange choice to be on the list, but theres a lot to blastoise. Its very good bulk with mono-water typing makes it quite steadfast, and its ability basically grants it a STAB aura sphere and dark pulse, which hits relatively hard coming off of 135 special attack. Access to rapid spin makes it suitable for many teams on top of being a special breaker)
C- Rank

Mega-Manectric

Salamence (Less capable flyinium-z sweeper than dragonite and gyarados, but its typing + intimidate can occassionally give it the edge over the other options)

Thundurus

Ninetales-Alola

Moltres (Crippling SR weakness holds back this great type combo and moltres from having a great defensive niche in OU)

Mimikyu

Ribombee (Its probably the second best sticky web setter, and is cool for its fake out immunity and stun spore)

Alomomola

Pyukumuku (Has a cool niche on hard stall teams that want an unaware PP stall mon)
D Rank

Mega-Steelix (I can see a mega steelix team given the right support, but its a lot of work just to make it average)

Shedinja

Mega-Beedrill (Looks great in theory, in practice... adaptability boosting horrible stabs cant make them good. Being a mon thats exclusively used for pivoting while also being weak to SR is a death sentence too)

Forretress

Mega-Houndoom

Mega-Pidgeot (Unfortunately just kinda sucks, outclassed by torn-t in most ways except power and a consistent hurricane. At least its consistent in its mediocrity)