My opinions about viability have changed fairly substantially since my last viability estimates rankings, so I think this is due for an update. Do note, nothing here is in any specific order unless otherwise specified.
S:

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What can I say? Birds fly, frogs ribbit, and creatures made of metal beyond human comprehension do whatever they do. I think these three hold a very special place in SS OU. Tornadus-therian never dies; Heatran has too many sets; Melmetal is difficult to kill.
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I don’t feel I have to explain any of these- other than Garchomp. Garchomp doesn’t and won’t dethrone Landorus, but its performance in the last year was more than enough to consider it on-par with the rest of S-.
A+:

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Again, these are self-explanatory.
A:

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I really want to emphasize Volcarona, Tapu Fini, Corviknight, and Dragonite here. I was very impressed by the performance of SubSwarm Volcarona as of late, especially with Hatterene, which I will cover later. Tapu Fini is a rather solved 'Mon, but invested CM LO + Hydro Pump caught my eye on several hyper-offenses. I am not a fan of Corviknight personally, but NBM solely convinced me that it has a serious place on successful teams even if it is just a passive bot. Dragonite- and I want to emphasize this- is LOW in A-tier. I like Dragonite's defensive and offensive profile into offense teams, but into everything else it's just so mediocre. Contact effects this, Rocky Helmet that, Weavile, Garchomp, Rotom-Wash, Corviknight, Ferrothorn, Scarf Lele, Air Balloon Heatran, Buzzwole, etc. the list is so exhaustively against Dragonite's favor.
A-:

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Okay, so, three figures I want to focus on: Hippowdon, Zeraora, and sand. Firstly, Hippowdon has had a generational run in tour recently, and I personally have been enjoying it more on sand teams with Excadrill than Tyranitar. Secondly, my opinion on Zeraora has substantially changed. Its performance has been rather positive, and for me personally, variations of Bulk Up such as LO + Play Rough have made it a better endgame choice into Garchomp and Buzzwole. Lastly, Tyranitar. Tyranitar, by contrast to Hippowdon, has been having an awful run. I haven't seen much much if any offensive sand, and where I have seen Tyranitar it's been Braziltar rather than Bandtar. We all know how terrible that set is. I actually would have dropped Tyranitar lower if it weren't for Choice Band and its association to Excadrill on sand as an archetype, so it narrowly dodged B+.
B+:

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Nothing much to say about these, fairly standard takes.
B:

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My apologies Hatterene, I was unfamiliar with your game. Seriously, the amount of times Hatterene appeared in tours recently astonishes me. Magic Bounce HO as an archetype in SS OU feels very different from USUM, but the threats really haven't changed (i.e. SubSwarm Volcarona, LO Weavile, etc.). My opinion about Suicune was also changed as of late, having seen its performance in tours. Turns out clicking Substitute, Protect, and Scald doesn't get old.
B-:

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Choice Specs Hydreigon is a menace, I mean that sincerely. As for Mandibuzz, that one balance team featuring it has done numbers sheerly because Mandibuzz does nothing all game but click Roost and have a good defensive profile. Sound familiar? Now you might be asking, why is Latios ranked so far from Latias? The same reason Hydreigon is ranked in B-, Choice Specs. Latios also has a better speed profile which is great into Banded or SD Kartana. Draco coming off of Latios chunks steel-types, while its coverage moves typically threaten 1HKO or 2HKO on other targets.
C+:

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I don't have much to say about C+ other than the seemingly VERY low rankings of 'Mons I previously ranked higher, or would have ranked higher. Those include Keldeo, Primarina, Cresselia, Nihilego, Mamoswone, and Seismitoad- basically almost all of C+. Keldeo and Primarina are grouped pretty closely, they're both water-type special-attackers that do well on paper but perform under par. Primarina specifically, since it has the same defensive profile as Tapu Fini, but with one major drawback: low physical defense. Primarina wants to invest in its naturally higher special-attack stat, but with uninvested defense and without Leftovers, it can't actually check things like Urshifu and Weavile. Cresselia gained some traction recently in tours, accompanied by Suicune, but I'm not convinced just yet, no more than I am Reuniclus. Nihilego and Mamoswine fall into the category of "I can either snowball an entire team or get stonewalled and there is no in-between," so I'm less keen on them than I used to be. Finally, Seismitoad. Rain simply isn't that good anymore, I see it in tours, I see it in casual matches, I build rain, and I have to say that I'm just not impressed with its performance. That said, I'm really not impressed with Seismitoad either. You need really dedicated rain to use it, and honestly, a frail Zapdos check is not a good Zapdos check. Causes more problems than it fixes in the builder more often than not, aside from role compression on Melmetal rain.