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 (Genuinely the most splashable mon in the tier)

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)

Heatran

Mega-Charizard-X (Not only is an amazingly powerful sweeper on HO, but has a great deal of value as an SD breaker on slower paced teams)

Chansey (Cornerstone of balance and stall, the tier would fall apart without chansey)
Cresselia (Super versatile, most sets can destroy offense teams and it can work as a lone check to things like mega-alakazam, mega-medicham, tapu lele and more)

Serperior

Tapu-Koko (Koko hate is so forced. 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 checks to it, both common and niche, into use. You'd be hard pressed to find a good team without at least 1-2 ash-gren checks)

Mega-Charizard-Y (Has the least switch-ins out of anything in the tier... great speed tier for how hard it hits, and Roost + Good Natural Bulk grants it more longevity 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 staples, but a bad predict against it can lose the game instantly ─ great on balance teams as an anti-offense pick)

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)

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 with amazing stabs and coverage. Unfortunately, its quite limited to HO teams because it has no longevity and not the best defenses)
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. Still very strong, and a great sand enabler)

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 electrium or groundium Z sets are great for breaking past toxapex and celesteela)

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 boosted sweepers and things like Mega-Lopunny)

Garchomp
B+ Rank

Ditto

Victini (A fairly good scarfer because v-create makes it very powerful despite its lackluster attack, but being weak to stealth rock leaves it lacking as a pivot, and sacing itself turn 1 with final gambit is often more value)

Celesteela

Magnezone (Fairly one-dimensional as trapping is its only niche. Despite its predictability, setting up a trap isnt too difficult and eliminating steels enables so many things like Kartana, Mega Pinsir, Alakazam and many more)

Zapdos

Mega-Gyarados (Quite a potent sweeper against more slow paced teams, as substitute or taunt can shut down defensive counterplay, as well as mold breaker ignoring unaware, magic bounce (for taunt variants) and wonder guard)

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 (Super underrated special wall, better than chansey in many ways due to being able to pursuit trap psychics and spam knock off against defensive teams. Being poison allows it to absorb t-spikes as well)
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 (Most of the time is just a big MU, cant break balance that well because setup latias and gliscor are quite common and beat it. A good check to offensive psychics, especially medicham)

Buzzwole (A very underrated pick as a physical wall, but it is a great check to many common physical sweepers in the metagame. It can shut down many things like Kartana, Mega Scizor, Garchomp, Mega Tyranitar, Mega Gyarados, Gliscor, and its a nice check to pretty much all physical attackers)

Keldeo

Tyranitar

Mega-Sableye (Stall isnt that bad of an archetype, it just requires more innovation than other archetypes because of the sheer number of wallbreakers. Mega sableye is almost required on any viable stall team, although it is limited to 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 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 (Great cleaner because of its STAB knock and sucker punch. Utility sets with pursuit arent nearly as good as sweeper sets though. Also a must-have on web teams.)

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)

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 (Pretty good sweeper, +6 aqua jet hurts, and still sweeps most weakened teams)

Mega Heracross
C+ Rank

Blacephalon

Mega-Sceptile (Very good into offense, leaf storm OHKOs stuff like tapu koko)

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

Mega-Ampharos (Volt switches are strong af, and this is the only mon where its HP ices actually OHKO lando and gliscor)

Porygon-Z (+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 (Best sticky web setter in the tier, also decent as an offensive mon on rain)
Ribombee (Its probably the second best sticky web setter, and is cool for its fake out immunity and stun spore)

Marowak-Alola

Mega-Blastoise (Its very good bulk with mono-water typing makes it quite sturdy, and its a fairly strong special breaker combined with rapid spin utility)
C- Rank

Mega-Manectric

Salamence (Less capable flyinium-z sweeper than dragonite and gyarados, but its typing + intimidate can occassionally make it worth it 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

Alomomola
Mega-Beedrill (While adaptability u-turn is strong and it has good utility in knock off and toxic spikes, its stab typings are quite awful, and it is very vulnerable to rocky helmet and SR chip)
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 on par with other bulky steels)

Shedinja

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

Mega-Houndoom (Could possibly be decent on sun teams with torkoal/ninetales)