Meh, I feel like talking about some mons quickly, and the viability ranks are looking more than a little skewed (We have a total of 6 mons in A- and 32 in C+, to look at the extremes. C+ is probably over twice as big as literally anything else, and A- is probably half the size or less of anything not named S. So, there's probably some room for adjustment to balance things a little.) anyways, so.
Let's start with the easy one(s).
Avalugg to C-/D
Okay, am I missing something? Was there a reason this was ever that good other than it was beating all the annoying offensive mons in the trash meta that was Sneasel/Gallade? It's a specially frail defensive ice type that's slow as balls, a spinner weak to rocks, and it became relevant because Sneasel was on every other team. I mean sure, the physical bulk gives it a niche. It's still really not that good anymore.
Ninjask to C
It's certainly not a terrible pokemon. Ninjask however has a few things it struggles with. One, is a 4x weakness to stealth rock, lowering the amount of times it can come in if rocks are on the field drastically, especially when it wants to click U-turn like 95% of the time. It's got niche over it's Bug/Flying big brother, Scyther, but it still suffers from heavy competition from it. In fact, it's niche basically comes down to having a stronger u-turn than scarf Scyther(Random calcs to give you a picture of comparable hits between the two:
252+ Atk Choice Band Ninjask Aerial Ace vs. 152 HP / 0 Def Fletchinder: 204-240 (63.1 - 74.3%) -- guaranteed 2HKO
252 Atk Technician Scyther Aerial Ace vs. 152 HP / 0 Def Fletchinder: 211-250 (65.3 - 77.3%) -- guaranteed 2HKO
252+ Atk Choice Band Ninjask U-turn vs. 248 HP / 252+ Def Vileplume: 118-139 (33.4 - 39.3%) -- 15.7% chance to 3HKO after Black Sludge recovery
252 Atk Scyther U-turn vs. 248 HP / 252+ Def Vileplume: 82-97 (23.2 - 27.4%) -- possible 5HKO after Black Sludge recovery)
and a much better speed tier than banded Scyther.
With steels being high in popularity (Klinklang and Mawile to be specific), bulky poisons like Weezing and Garbodor being popular as always, (the latter of which has rocky helmet typically), bulky ghosts like Misdreavus and Golurk rising in popularity, and bulky defensive mons that can sponge hits from a Ninjask easily, ie Rhydon, Carracosta, Regirock, Piloswine, Ferroseed, and various other checks such as Fletchinder, Rotom, Lanturn, Magmortar, Archeops and Aurorus being common, Ninjask has a lot to struggle with, and though it can spam U-turn and get momentum, it also has to struggle hard to even make it's way onto a team over Scyther just to be middling against a lot of pokemon. Scyther also shares problems with these pokemon, but both has a bulky set that manages better against some of these mons, doesn't have to suffer competition over something that does most of what it does better. I suppose it can't pass SDs + Speed anymore either, but that set sucked anyways so who cares there.
Dusknoir to C-
Can I just say, that this thing has been overrated for a long time. It has exactly one viable set; Banded. This set has a large amount of flaws. First off, the obvious, competition. Golurk hits a lot hard, has STAB EQ, Iron Fist to boost moves(including pathetic STAB otherwise), and also No Guard for various uses. Dusknoir has Shadow Sneak, Trick, and Pursuit, but it's both incredibly weak(it's strongest move is base 100, nonstab, out of anything it's be running, which automatically means it's not doing much damage), and incredibly easy to take advantage of, and if it locks into any of it's options, there are many set-up sweepers in the NU metagame that can easily take advantage of it. It can trick, but then it becomes even more dead weight offensively. The only thing really in it's favour is that it's a bulky spinblocker, but lately another bulky ghost, Misdreavus, has came into favour, and it's a lot more useful than Dusknoir. Heck, there's even Gourgeist that stacks up well against most of them.
Vullaby to C
Vullaby, while not the most impressive mon in the world, certainly capitalized from the departure of Togetic. Now, it and Pelipper are the only viable defensive defoggers with access to reliable recovery. It's also got a solid ability in overcoat, allowing it to stop Lilligant, impressive bulk after eviolite, options to run both physically and specially defensive, the ubiquitous Knock Off with STAB, STAB Foul Play giving it some offensive presence against a variety of mons, and taunt utility. Defensively, while it does have weaknesses to common Electric, Ice, and Rock types, it can usually chew hits from even these pokemon, and it also has resistances to common dark, ghost, and grass types as well as immunities to ground and psychic. It's also neutral to the common signal beams, which allows it to check most psychics fairly well. A speed tier of 60 allows to to outspeed a lot of common mons and drop taunts on them as well.
There's also potential to move Kadabra up to like B- I feel, as it's a very unique mon with a solid speed tier, movepool, and ability, and sashed Kadabra is a really cool 'safety net'. That being said I'm not that experienced with it so I'm not going to say much about it.
I also want to quickly mention Victreebel again, as while I feel personally it doesn't need to make a huge raise, I think it more than deserves to at least B-, as it is a gigantic threat, even if it is nowhere near Ludicolo's level, but I mean Ludicolo is sitting pretty all the way up in A.
Out of what could jump in C+ I'll also say Simipour, which sits at an interesting speed tier of 101, has nasty plot, allowing it to be a huge threat, water/grass/ice coverage unresisted by anything not named Sap Sipper Marill, focus blast to bop Ferroseeds even, and potential for more gimmickys sets involving things like physical moves with it's statspread and movepool.
Also finally I feel Stunfisk dropped too far, as it stops Klinklang, provides paralysis support, has unique typing, a lot of bulk, and threatens contact attackers just by them touching it. Might not have the best resists, but I feel like there's more than a small case it could go about B-, though I'm too lazy to make one.
Okay, now moving onto some better mons.
Poliwrath to B+Even with Sneasel gone, Poliwrath is still a very solid mon. Special is easily it's best set, though even that can be versatile, as it can be ran bulkier, faster or even mixed(Hi Lanturn, my name is Earthquake, get 2hkoed.
4 Atk Life Orb Poliwrath Earthquake vs. 40 HP / 252 Def Lanturn: 198-234 (49.3 - 58.3%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery). Special 'Wrath is a rare user of the move Vacuum Wave, a solid priority move allowing it to pick off fast threats. It hits hard with powerful dual stabs in Hydro Pump/Scald and Focus Blast, has amazing typing, an okay speed tier, and coverage in Ice Beam and even Psychic potentially. It also has other sets, however. It run a defensive phazer with circle throw, which is able to check a lot of threatening sweepers and such, subpunch with stab, etc, with plenty of room for innovation and creativity on any of it's sets. Even the standard can certainly be played with right down to EV spreads. It's somewhat low stats might seem a bit underwhelming on paper but it's typing and moves give it a lot more utility and viability than at first glance.
Golurk to B+
Golurk has always been an interesting pokemon. It's unique Ghost/Ground typing gives it some interesting typing quirks, with immunities to common Fighting, Normal, and Electric pokemon and a resistance to Rock. 124 attack is amazing, and 89/80/80 bulk is decent. It's able to set rocks, poke holes in things with Iron Fist boosted punches and STAB Earthquakes, has a great offensive movepool including Fire Punch, Ice Punch, Thunder Punch, Shadow Punch, Earthquake, Zen Headbutt, and Drain Punch, has No Guard DynamicPunch and Stone Edge, and even Rock Polish for potential use. It's speed stat is underwhelming, but when invested it can still outpace a lot of mons, and it can chew hits from even more. Ghosts in general have improved in this meta.
I assume Mismagius fell due to Sneasel, but I think there's more than enough of a case to bump it back to the A ranks, at least to lonely A-. It's got great sets, being able to run both Nasty Plot and a defensive set featuring Taunt and Wisp. It hits hard, has solid special bulk, a great speed tier, and it's improved a lot in this metagame.
Maybe this one's a little crazy, but I'm also gonna nom Claydol to A-. Claydol is easily our second-best hazard remover, and also a more reliable spinner than Kabutops, even if it doesn't bring as much offensive presence. Claydol has good bulk, an interesting speed tier, and an amazing special movepool, allowing it to beat a lot of common spinblockers, keep offensive presence, chew hits when needed, check certain mons, and also beat a fair amount of rock setters. Levitate and it's typing gives it a resistance/immunity to edgequake, fighting, and electric, giving it some decent typing quirks despite it's weaknesses. It's fast enough to outpace the base 70s and Malamar speedwise when invested as such, can be played with in spread, movepool, and even item(can run LO, Lefties, Colbur, even some other random resist berry for like Grass, Water or Ice probably, and I'm sure there's probably even more that could work), can set rocks if needed, has Signal Beam, Grass Knot, Ice Beam, and Shadow Ball among other things in the coverage department, rare special ground stab, and can check a lot of things, even such as Sawk(it can chew a hit and be running enough speed to outspeed it if adamant band,) Mesprit, or Rhydon.