I'm sure most of it's been said already, but I've got some noms and agreements:
RISES
A -> A+
Fini has really come into its own in this meta. The Whirlpool set is downright nasty, especially with Knock Off or Knock support to drop Pex's Shed Shell. Scarf is a great Weavile, Urshifu, and Dragapult revenge killer for offense and a fantastic disruptor with Trick and potential Taunt or Defog, and Calm Mind is a MU fish but can be a great wincon and particularly good Urshifu and Weav check. Plus terrain support is very nice for teammates like Garchomp and Zera. This mon is bona fide top 10 right now, it deserves A+ as much as anything.
A- -> A
Scizor has truly surprised me with just how solid it is as a defensive pivot. It gives awesome role compression as a switchin to Specs Kyurem's and Tapu Lele's STABs, a Sub Kyurem counter which is very VERY important to have, and a soft check to a bunch of physical attackers like Rillaboom/Bulu, Weavile, and Melmetal, and it can even clean lategame in some matchups. Of course it can get overwhelmed if it has to fill two or more of these roles at once, but it's no different from other mons with similar role compression like Lando-T or Fini in that regard, and unlike them it can Roost it off. Overall, Scizor's been a force in the meta and very much deserving of A tier. TL;DR: I concur wholeheartedly with all the points
agslash23 and
Windingsss raised
B+ -> A-
Suffice to say, there's not another Volt Switcher that does it as well as this thing. It pivots all over the many, many teams that use Landorus-T as their Volt absorber, invalidates its Grass-type checks like Ferrothorn and Amoonguss by bringing in teammates like Blacephalon and Kyurem on them, making it one of the best ways to support them and other hard-to-get-in, high-reward breakers. On top of this it checks Melmetal, Dragonite, Heatran, and even Cloyster. A good VoltTurn build is often made better by using this thing, it's very underrated.
B+ -> A-
Bulu's proven itself to be a menacing breaker and tank as of late. Buzzwole and Amoonguss are generally the only Pokemon that can switch in on this thing safely; even Iron Defense Skarmory and Corviknight aren't safe if they switch in on Swords Dance. Access and resistance to Close Combat, Stone Edge to obliterate Tornadus-T and Volcarona, and actual recovery in Horn Leech make Bulu, in my opinion, firmly better than Rillaboom and give it a strong presence in the metagame.
B+ -> A-
Victini's finally found its place in the meta, and it's just as threatening as many predicted it would be. It forms fantastic pivoting cores with top tier Pokemon like Landorus-T and Tapu Koko and takes advantage of the general lack of Pokemon that can handle its coverage, especially in Electric Terrain. Along with its bulk and typing that help it ease matchups against Lele and Kyurem, this makes Victini a very good Pokemon currently.
B -> B+/A-
Blacephalon is the rising star that nobody saw coming. This Pokemon's been highly successful as a breaker with how few "checks" can take its Specs Overheats and repeated Shadow Balls, and those that can, like Blissey and Tyranitar, just get Tricked. Basically everything
AquaVanilla,
BreakthrU89,
ausma and others have said I agree with.
SEND IN THE CLOWNS
B -> B+
Ghosts really have had a lucky break lately. Aegi is a very cool mixed breaker right now with Life Orb or Spell Tag, and I've heard Specs sets have seen success as well. Like its buddies Pult and Blaceph, it struggles to break Toxapex, but drops from Shadow Ball help remedy that issue. It's not the most consistent pick and often relies on heavy prediction to break bulkier teams, but it deserves a rise because it offers plentiful and relevant offensive and defensive qualities over other similar attackers.
C+ -> B-
Did this thing drop? I don't see why; VinCune is certainly still a threat and probably one of the most annoying Pokemon in the tier to beat. Its Subs are really hard for most of the defensive meta to take down, and even when it doesn't sweep it stalls out moves like Urshifu's Close Combats and Dragapult's Draco Meteors for its teammates and can check Cloyster, Volcarona, and Weavile in emergency situations. If nothing else, it's surely a step above the other more niche Pokemon in C+.
C -> B-
Not much to be said here that hasn't been said a thousand times. Please ban King's Rock so we can get this thing off of the rankings and out of our heads.
C -> C+/B-
Hail is becoming increasingly common and successful, especially on ladder, with Arctozolt as the face of the playstyle. With hail and veil support from Ninetales, this thing shatters a lot of teams with its STAB combination alone. The inconsistency of the playstyle and Zolt's relative uselessness against offense outside of hail are concerns, but Zolt's proven itself more than potent enough to be worthy of a rise.
C- -> C/C+
We've all smashed our heads into our keyboards facing Shed stall at one point. Boots and the loss of Pursuit were huge buffs to this thing, since now stall teams can properly get it on and off the field. With sand less common than ever, Shed lends itself to being less of a matchup fish and an increasingly valuable asset to teams that need a Pokemon to reliably wall Tapu Lele, Kyurem, and Future Sight + Urshifu, and stop the latter along with other threats like Koko and Skewda from spamming their pivoting moves. Shed is still niche for obvious reasons, but it's proven to be a surprisingly viable defensive threat that solves many of stall's worst matchups, more so than a C- ranking would indicate.
DROPS
A+ -> A-
Gomi described the fall of Slowking better than I ever could. The Pokemon that it checks have adapted to it extensively; it's too easily overwhelmed by them to carry out its other duties consistently and is extremely vulnerable to getting knocked and statused by the few Pokemon it can generate free turns against.
A- -> B+/B
Hoooo boy, big Bish. This thing's had a pretty steep decline recently mostly because consistency is nonexistent for it; Band and SD sets alike are almost entirely matchup reliant to do what they need to; if the opponent has, for example, a Skarmory, Urshifu, or especially Buzzwole, the game becomes a serious uphill battle for Bisharp. Plus there's a certain other STAB Knock Off sweeper rising in prominence that puts Bisharp on the back foot. Bish has had its fun, but it's likely time for it to drop down the ranks again.
A- -> B+
Blissey struggles. Hard to put it any other way. Thing is too passive and gets U-turned or Volted on by everything it actually checks well i.e. Zone, Koko, Pult, and Rotom-W. A lot of the other special attackers like Lele, Heatran, Hydreigon, Blaceph just trap, Trick, or bust through it. Plus it's not a great Future Sight absorber because it has no passive healing and it can't absorb the physical hit that often accompanies FS. This thing has far too many flaws to sit in the A ranks.
A- -> B+/B
I'll be honest, as someone who used Zap all the time in gen 7 I haven't vibed with it at all this generation. Torn-T outclasses it wholly as a Defogger, caring decidedly less about Knock Off, outspeeding the Pokemon it wants to check like Kartana and Urshifu, and being less passive with a Knock Off of its own and U-turn. Better bulk and typing means Zap can get the job done in a vacuum but it's just so vulnerable to Knock Off, and doesn't check anything very consistently outside of Melmetal as a result if it can't get a Static para. I certainly don't think Zapdos is better than Mandibuzz in a defensive role, so to B+ or B it should go, IMO wherever the vulture ends up.
B -> B-
This one is quite frankly unavoidable. Can't believe I'd see the day Excadrill is legitimately just straight up bad, but here we are. It relies entirely on Iron Head/Rock Slide flinches to break anything, since it gets blanked by half of the defensive meta, it's worn down super easily into range of stuff like Urshifu Aqua Jet, it has very little defensive utility because it's a Ground-type that gets OHKOed by Zera and a Steel-type that gets blasted into the stratosphere by Lele's STABs, and above all it's totally useless outside of sand. With every new meta development, poor Drill keeps digging its own grave lower and lower.
B -> B-
Oh boy, a bulky Ground-type with no defensive utility! Swampert is just too slow and can't make progress against anything, it's total U-turn and Knock fodder sine it's super passive and can't heal. This thing holds the distinction of being the bulky Ground-type that loses to Zera the quickest, go figure. It's a cool rocker on Grass spam teams since they can cover its lack of recovery with terrain and they tend to be super weak to Heatran, but other than that this thing is totally outclassed by SpDef Lando-T.
B- -> C+
In theory, this thing breaks and cleans up nicely with Speed Boost. In practice, the only thing sped up is its death after 2 turns of being on the field. Just use Zera for a sweeper that's fast out the gate or Victini for a Fire-type physical attacker. Admittedly it's still cool on screens, which is niche to begin with, so as such it should be chilling with Grimmsnarl in C+.
C+ -> C
Rain is a niche playstyle to begin with, but when was the last time someone actually used Kingdra? Barraskewda and Seismitoad outclass this thing as a Swift Swimmer because of the former's strength and asinine Flip Turn spamming and the latter's defensive utility, coverage, and access to Stealth Rock, and Volcanion outclasses it purely as a rain special attacker because it's an absolute nuke. With SpDex Toxapex, Fini, Ferrothorn, and Glowking on the rise, Kingdra's gonna inevitably struggle to make its mark on most teams.
C- -> UR
I have not once seen this Pokemon in the past 9 months in OU. It has no reliable recovery, it's taken advantage of by pretty much every other defensive Pokemon and every special attacker because it's so passive and specially frail, it has 4MSS because it needs to run Pain Split and one of its easily wallable STABs and then it has to choose between Flamethrower, Will-O-Wisp, Defog, Toxic (Spikes), Aromatherapy, etc. If it could run all of the aforementioned moves on the same set, maybe it would be decent. But as its stands, there's basically no use for this thing and it checks nothing that Buzzwole, Corviknight, and Skarmory don't check far better.
C- -> UR
I can understand the rationale for this being ranked, at least more so than Teddy Roosevelt's asthma that I've discussed above, but realistically Alakazam never gets a chance to set up. Its bulk is totally abysmal, it's outsped by 5 of the most common threats in the metagame that can all take advantage of it, and it needs Life Orb to do any sort of damage even at +2 (for reference, it has no shot of OHKOing Tapu Fini or SpDef Lando-T after Rocks without it), which means even Toxapex can hit it with Knock Off and essentially neutralize it. Its reliance to break Steel-types with Focus Blast also hurts it immensely, since it means there's always a chance something like Melmetal could sidestep a Blast and blast back, rendering Zam even more useless. Zam is just not strong, fast, or reliable enough to offset its terrible vulnerabilities, to the point where either C- should stay its permanent home or it should be orphaned to UR.
Other noms I agree with:
S -> S-
A -> A+
A -> A+
A -> A+
A- -> A
B+ -> B
(B specifically, hard disagree with B-. While it's true Mandi is very vulnerable to Knock Off and Toxic, being a Defogger that can switch into both Pult and Kart means Mandi can still pull its weight on the right team.)
B+ -> B
B -> B-
B- -> C-
Noms I disagree with:
A+ -> A
(Suppose I could go either way on this one, but I lean more towards low A+ because Bulk Up Zera is just incredible.)
A+ -> A
A- -> B+