C+ -> B
Strongly support this. So as to avoid redundancy, I'll use this reply to specifically explicate the niches it holds over Tangrowth and (to some extent) Ferrothorn and why these quirks are meaningful.
Unlike Tangrowth, it sports a favorable matchup vs Tapu Fini due to Sludge Bomb. The fall in popularity of CM sets in favor of Whirlpool and TrickScarf make Sludge Bomb better than Clear Smog these days but if your team doesn't naturally have a CM Fini check Clear Smog is pretty cool, missing the 30% poison chance makes you significantly more passive though. Also unlike Tangrowth, Amoonguss's Foul Play means it does not need to rely on a Focus Blast connecting to beat Kartana; should Focus Blast miss vs a +2 Kartana, Tangrowth is cleanly 2HKO'd by Knock Off into Sacred Sword. And of course unlike Ferrothorn, Regenerator makes it far safer to use as a mid-ground over the course of the game.
As HydreigonTheChild mentioned, Foul Play Amoonguss is a better blanket option for physical set-up sweepers; it is risky since it loses to, say, +2 Garchomp unlike Tangrowth, but Tangrowth itself is a risky option since you are relying on Sleep Powder and a long period of sleep to switch to an offensive check assuming you cannot safely 1v1 with Giga Drain. Amoonguss's lack of 4x weakness to Fire and good special bulk relative to Ferrothorn and Tangrowth respectively also makes it a midground for Dragapult and Aegislash's Shadow Ball. Assault Vest sets designed to be Knock Off absorbers with Spore perform this role exceptionally well due to Regenerator trivializing the damage taken — a rare trait in the tier right now — though are a bit more gimmicky/surprise-factor-y compared to Black Sludge or Heavy Duty Boots.
Moreover, Spore is a significantly more consistent option for forcing progress by always shutting down switch-ins and potentially forcing telegraphed switch-ins into Spore absorbers which can give you the initiative with a well-timed double, A lack of Knock Off relative to Tangrowth and Ferrothorn is a real bummer since it forces switches with Spore, but fishing for Sludge Bomb poisons is a reasonable consolation prize that make it a pain to switch into.
tl;dr: Though it lacks the impenetrable physical defense, access to Knock Off, and freedom to run Rocky Helmet compared to Tangrowth, it compensates with more balanced defenses, a better sleep inducing move, a more consistent matchup vs physical set-up not named Weavile, a good Tapu Fini matchup, and neutrality to U-Turn. It does not have the same degree of non-passivity compared to Spikes Ferrothorn, but it has Regenerator for more longevity to continue to keep its checks in place and the freedom to spread status. I truly think this 'mon is criminally underrated and compresses a lot of roles into one (Grass resist, Shadow Ball midground, Shifu check, Zeraora check, physical set-up check), and I agree that it deserves a raise.
Arctozolt C -> B
-> A: Fantastic Knock absorber and blanket check to most physical attackers in the tier thanks to its Roost access, impressive 107/139 physical bulk, and reasonable defensive typing which gives it crucial resistances in Fighting, Ground and Dark. Its 139 attack stat and great coverage options make it far from passive as well. Granted this thing is completely helpless versus any Flying type in the tier due to its 4x Flying weakness and horrendous special bulk but that doesn’t negate the fantastic defensive and offensive utility this weird mosquito looking thing has to offer.
-> S-: Very dangerous cleaner/breaker with access to 2 very strong STABs in Ice and Dark as well as a high base Speed stat and access to priority in Ice Shard. However it isn‘t quite as splashable as Pult or Lando-T and like Heatran its 4x weakness to a common attacking type (Ice/Dark as a whole is pretty poor as a whole defensively even barring the glaring 4x weakness) holds it back somewhat.
-> B-: Very cool mon on paper, in practice it doesn‘t have enough longevity to do much of anything on any team archetype that isn’t Grass Spam (which to be completely fair it is solid on). Please let this thing drop to UU.
-> B+: Spreads status well and completely walls most special attackers but is fairly passive and the rises of Shifu-R and Weavile (not to mention Pult becoming much more manageable) have really hurt it. Edit: Also top-tier special attackers (Pult and Heatran) have ways around Blissey, Pult just U-turns out while Heatran’s combination of Taunt, Toxic, and Magma Storm allow it to trap Blissey, prevent it from using any of its utility moves, and wear it down with Toxic.
-> A+: Misty Surge is nice and the Scarf set provides great speed control capable of checking the aforementioned Shifu-R and Weavile.
-> S-: Pult has undeniably fallen off somewhat but it’s far too splashable to go anywhere other than S IMO, it’s still a very fast mon with a wide movepool and great STABs. The recent rise of Specs Blace has also helped out Pult a lot.
-> A-: If this thing didn’t get BU I’d agree with a drop since it hits like a wet noodle without boosts but the BU set is currently A+ worthy IMO
A -> A+
A -> A+
C+ -> B-
C+ -> B-
C+ -> B-
C -> B-
C -> C+
C -> C+
C -> B-/C+
C -> B-/C+
A+ -> A-
A- -> B+
B+ -> B/B-
B -> B-
B+ -> B-
B- -> C+
B- -> C+/C
C- -> UR
--> A. There's a lot of role compression in Scizor and I absolutely adore his current sand tomb set to deal with some potent threats such as heatran. While not usually the feature player on a team, it provides an excellent partner to many a mon.
--> A. I think everyone else will disagree, but I think the boom deserves the slight bump to sit among the A tier mons. Although it has a lot of threats, Grassy Glide go Brr is actually still a very strong priority move. It can OHKO Weavile (it survives an ice shard, even with a SD and glide has prio over Triple Axel if they do not switch) and Urshifu and has a sneaky OHKO on Pult with Knock Off if they are locked out of Draco Meteor or with prediction. I've also been a big supporter of dropping Wood Hammer and adding Superpower, where it can then OHKO common switch-ins like Heatran. It still has some big weaknesses and checks, but I think the mons in the A tier all have similar weaknesses.
--> A-. I love Mew. If you read my comment, you would see that I highly value versatility, and Mew has that in abundance. You can do nearly everything utility-wise with this guy. Also Mew is hands down one of the better leads in OU.
--> B+(with Blacephalon possibly going A- soon even!) Ghost has always been strong, but the type finds itself in a very good place in the current meta. There really aren't that many checks, and the few there are can even be dealt with by these 2 ghost mons, albeit not as consistently.
--> B+/B. It really isn't that bisharp is bad. Actually, bisharp is quite good. It mostly is just the fact that there's very little reason to run it. Weavile is heads and shoulders above bisharp right now and it is going to take a really specific team to justify using this instead of the latter. Sometimes a mon doesn't need to be worse because of itself to drop, just because of a better option.
--> B-. When you can't get it done vs what I would consider the top 3 offensive threats in the game (Weavile, Pult, and Urshifu) you can't really justify being high on the Viability Ranking. Not to mention Kartana and Kyurem also checking it. Taunt has seen plenty of use as well. There really isn't much going for this guy unless everything goes his way.
--> C+. I think it's been said before but Rain is relatively weak in the current meta. I love the archetype, but this is just not the time for it and the VR should reflect that. It is no better than Sun and I could even make the argument that Sun is maybe a bit stronger.
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--> C/C+. Ok hear me out. Trick Room is a little viable right now. I know it's always been a gimmick, but it's actually started to work. It was evident in OLT, but you are seeing it on the ladder more. It isn't amazing, but you have to include it now. I am not including the more gimmicky 
here, I do not think that's what made the Trick Room great. But I have to acknowledge its viability. Maybe it is just me, but I think raising them all to sit with Cresselia in C is safe, while an argument for C+ can be made. 
can also be considered here.
--> A-/A(maybe)
--> B+
--> B-
-> A. 
-> A+. Kartana may be one of the best wallbreakers in the metagame, but frankly I think its fine where it is. This is because Kartana is prediction-reliant and doesn't like being locked into any move. Further, it is very easy RK fodder for Dragapult, Zeraora, Tornadus-T etc.. These traits mean that it has switch out plenty of times in the game, which hands over momentum and initiative to the opponent. While it breaks walls like none other, it gets manhandled by speedy teams. Scarf Kart exists to surprise these RKers, but doesn't hit as hard against walls, which makes it a mediocre set.
-> UR
C -> UR/C- 
to A+
to B/B+
I would love to hear your thoughts, but do consider the question I asked before I gave the reasoning.