First off I'll repeat some noms I still feel need to pass through:
SOME NOMS:
Mega Manectric to B-: With the opportunity cost rising, this thing has less and less of a place in B. It can't handle T-tar, it just straight-up loses to double defog stall, and rain does have Mega Swampert and Kingdra to just sit there and laugh at it, regardless of whether rain is up. In addition, competition is rising with Mega Gallade, Mega Zard Y and Mega T-tar improving. I don't see this as more effective than Zard X, Azumarill or Mega Shark, and I don't see it on the level of Terrakion, Mega Slowbro, Excadrill or Hoopa-Unbound.
Mimikyu to B-: This tends to only fit on SW, which is falling off rn. While it's still good on TR, TR is not enough to justify this in B any longer.
Mantine to B+: This thing is so useful to have on teams. It walls Zard Y and Volcarona and thus lets teams with it not have to run a >100 scarfer. It also takes on rain very well and can remove hazards for one's team (tho imo it's a bad defogger, it works) while burning mons with Scald. This is nice role compression that IMO deserves a rise.
Now, for the further scourging of the B ranks (meaning B+ and B- too)
Salamence to C+ - No reason to use this over Gyrados rn sans the 100 base speed - insignificant as it's gonna get revenged with the >100 scarfers anyways. It has a terrible defensive typing and not enough bulk, making it very hard to set up in this current meta.
Bisharp to B - Webs is dying and this was at its best on webs. Outside of it, it loses to bulky steels and it loses to rain. It also loses to the rising Tapu Koko.
Togekiss to C+ - Honestly, it's not very good right now. Its only niche is a stallbreaker, but it loses to Chansey, Clefable and even Zapdos without significant hax on its favor. Air Slash is a weak STAB move and there are plenty of flying resists or mons just flat-out capable of taking them on stall. I don't see this, a failing stallbreaker, as good as Ninetales-A, the heart of AV, Smeargle, the heart of the still-ok SW, Nihilego, a decent scarfer, Mega Gardevoir, a very strong breaker or Muk-A, a great Pursuit trapper and special tank. I also have no problem seeing this on the level of Mega Aero, Cofagrigus or Mega Chomp.
On a final note, I'd like to second Gastrodon to B rank because this thing truly walls rain.
And now it's time for the C ranks:
Dragonite to C - As a DDer it's just too easily neutered. You have >100 scarfers everywhere that'll outspeed it at +1. While it can get to +2 with its bulk, it needs SR off the field - not easy to do - and since it doesn't run Roost, it can be pressured and be prevented from setting up twice. At +1 it's just rather underwhelming. The opportunity cost of the Z-move slot is also too great. I consider Salamence, a mon I believe belongs in C+ and is a DDer, much better at its job with the potential to snowball out of control. I also consider Cofagrigus and Scolipede better sweepers. This doesn't seem out of place with Buzzwole, Cresselia, and Thundurus-Therian rn.
Gyarados-Mega to C - This just sucks. At +1 it can't really sweep and it's never getting to +2 with that god-awful defensive typing. The opportunity cost is rising with new megas being released. There's also an opp. cost compared to regular Gyarados, which honestly is just a much better sweeper. Honestly I'm even considering C-.
Breloom to UR - I haven't seen it at all this gen and it's not hard to see why. Terrible defensive typing, terrible bulk, terrible speed with Mach Punch being negated by Psychic Terrain, and Spore losing out with Tangrowth and Electric/Misty terrain everywhere. Rain outspeeds and kills, it can't really do much to bulky steels and double defog stall doesn't have issues at all. No reason to use.
Magneton to UR - With Magnezone falling off, I don't see why this is here. IK it's used as a choice scarfer that has an ok speed tier to beat stuff like gren but honestly it's supposed to trap steels and when it can't do that well enough anymore it's time for it to get UR.
Kabutops to UR - Mega Swampert just about 100% outclasses it.
The A ranks are pretty stable rn, no need to discuss changes there except for my previous two noms. I hate to seem rude or anything of the sort, but may I ask why Zapdos and Tyranitar weren't raised, particularly Tyranitar?
EDITS:
Salamence I agree. there is almost no reason to use it over gyarados.
Bisharp I think is fine in B+. Sure, Sticky Web is worse now, but Bisharp can also discourage defog in AV teams, which have been on the rise.
Kabutops has a niche in not using the mega slot, in case you want to use a steel type mega. Generally Mega swampert is the better option however, which is why Kabutops is in C rank
I agree with about everything else you said though.
EDIT: Gyarados Megas niche is that it can break past stall thanks to moldbreaker, allawoing it to easily dispatch Unaware clefable. In addition, it does not have to mega evolve right away so it can check Pokémon like charizard Y and Keldeo. Gyarados mega is fine in c+, it's never been amazing anyway.
I disagree with Kabutops being UR
Kabutops has niches over Mega Swampert, namely, Rock STAB, access to a Z move, not being a mega (Allowing you to run something like Mega Scizor), and acces to SD and Rapid Spin
Is still not as good as Mega Swampert, but that's why Mega Swampert is B+ and Kabutops is C
Mega Swampert is pretty much THE premier swift swim sweeper on rain teams for its defensive utility, sheer power and wide coverage. Honestly, Ground > Rock both offensively and defensively, and Mega Swampert's insane bulk more than makes up for both the mega slot and the somewhat lower speed. In addition, Offensive Z-Fly Lando is a great fit on rain teams and by using the Z-Move on Kabutops you're giving it up. Honestly I'd rather have those two than Kabutops.
Bisharp is gonna take me a long time to write up so I'll fully flesh it out later, but tl;dr rain's no good for it, Duggy + Zard Y is absolutely terrible for it and it alone could drop it to B, Koko beats it, bulky steels beat it. All of these are rising, and Bisharp just reacts terribly to these.
Gyarados-Mega has virtually no utility vs offense, as it's just too slow and has too poor a defensive typing. Against stall, it ails vs Skarmory (3hkoed but can roost away and phase out if you don't have Taunt), and Sash Dugtrio can use Reversal and OHKO. Honestly, I don't see it on the level of Mega Aero, which outspeeds almost every scarfer and can just straight-up sweep offensive teams late game, Mega Garchomp whose SD set murders any form of stall far better than Mega Gyara ever could, Manaphy which is a better balance breaker, or Scolipede, which uses Speed Boost to circumvent any revenge killers.
As for what got worse for it, it's the new stall. Double Defog just shits on it. If Zapdos gets in free and pulls off a discharge, Mega Gyara is going to take significant damage. If it's paralyzed, game over. Skarm, as I said, beats it, along with Duggy. Also, it can't break through Ferrothorn, anything offensive that's rising sans rain is bad for it. It's that bad vs offense. The release of Lopunny and Gallade-Mega hurt it a lot. Lopunny still outspeeds at +1 and kills, whereas Gallade provides competition by being better vs balance while still not losing to stall given that mega sableye is taken out for it.
MORE EDITS:
I think Salamence is fine in b-. Sure it receives massive competition from gyarados, but it's niche like you said was its base 100 speed, meaning it's not outsped by choice scarf lele. Salamence also benefits from the rise of mantine, which means less over base 100 speed scarfers for it to be worried about. While this is not enough to make it rise, it is enough for it to be just as good if not better than garbage like Smeargle
lol ok Smeargle is garbage now? It's still the face of SW and SW is still decent in the meta. Oh, let's go over some other B- mons too:
-Hawlucha can and will win games, but without taking up a Z-move and a lot faster too. It might not be as powerful as Salamence but it makes up for it with a +2 attack boost, much higher speed, high-powered STAB moves outside of a Z-Move and a better STAB combo.
-Muk-Alolan is a great pursuit trapper and special tank.
-Mega Gardevoir is a good wallbreaker that, given the support to beat Steels, can proceed to break the rest of the team's defensive core.
-Azumarill, another wallbreaker that has powerful priority to punish offense.
And then we have Salamence, a mon that will almost never sweep with such a poor base speed for a sweeper, that relies on a Z-Move to do anything and that has a terrible stab otherwise. I've never seen this thing sweep because stopping it is so easy. Just put a bulky steel in your team, or something bulky in general. SR is also a notable thorn because it's that easier to revenge. In addition, keep a fairy alive after the Z-Move and it's useless. Honestly, it's been pushed for months for this thing to drop and I fail to see why it hasn't.