Hey, I'll discuss then for some Top, Mid, and Low C-Rankers!
Top:
Archeops, Charizard, Haunter, Quagsire, Vanilluxe
Archeops is extremely deadly, as Flying Gem 140 Attack Acrobatics is extremely brutal. Stone Edge and EQ with that high Attack are absurd, and its solid 112 Special Attack allow it to pummel Steelix with Heat Wave or Rhydon with Earth Power. It is very frail and high maintenance, but otherwise has the potential to be a deadly Pokemon.
Charizard is a powerful sun sweeper, as STAB Fire Blast boosted by Sun and Solar Power is a pain and extremely hard to endure if your name isn't Slowking. Charizard can destroy whole teams with relative ease if given the chance, and also has an admirable Speed tier to complement his prowess. 4x weakness to SR sucks and he's really, really easy to wear down, but Charizard is still a lethal Pokemon.
Haunter is actually pretty good, with a good Speed tier (faster than any Ghost-type viable for use in RU, for the record), and has a solid 115 Special Attack to use some pretty strong attacks with. Haunter is really frail though so High C it is.
Quagsire is actually pretty good in RU tbh. Unaware is a great ability and allows it to deal with SD Absol and Gallade, as well as HC Durant and friends. His typing is pretty damn good to check Entei as well, so that's a plus. Nonetheless, Quagsire is a good asset to teams if you do happen to use him, but he's outclassed by Poliwrath so thus a Top C-Ranker.
Vanilluxe is pretty lethal with STAB Blizzard to abuse in hail, and it has some perks, such as a solid Speed tier to get at Absol while it has Autotomize to be really fast and spam mighty Blizzards. Faces really stiff competition from Glaceon and Rotom-F though and has lacking coverage, but it's good enough for Top C.
Mid:
Altaria, Drifblim, Electivire, Hariyama, Linoone, Primeape, Swoobat, Victreebel, Volbeat, Whimsicott
Altaria is a pretty good asset to defensive teams, and it has a solid Special Defense to be an okay check to Moltres and Sceptile (and I guess Slowking as well). Altaria is a decent special wall who also is a good cleric, and SpecsTaria is unique and can hit decently hard (Fuck Druddigon). Nonetheless, I think Altaria is a good asset to defensive teams as well as a decent Specs user and should be at least Mid C-Rank.
Drifblim is a solid Acrobatics user and can do decently well; and supports decently with D-Bond and can be an alright weather supporter. Drifblim can also use CM decently, although CMprit is better, Blim is pretty decent to use Shadow Ball and HP Fighting and can use Unburden to be a fast and powerful sweeper. That's about all there is to Drifblim.
Electivire gets a bad rap. It really does. Ever since his inception in DPP OU, this thing has always been mocked for one thing: He hits 13 of the 17 types super effectively, but despite that, he always misses out on KO's. That said, Electivire has a solid niche in RU as one of few viable mixed attackers in the tier. Its missing out on KO's can be patched up by layers of Spikes, and we have no shortage of Spikers either: we have many ranging from the speedy Scolipede to the bulky Roselia, so Electivire is always free to be paired with any Spiker. With Spikes support, a simple set of Wild Charge, Flamethrower, EQ, and HP Grass or Ice Punch can break down common cores like TanKing, SteelKing, or even EscavKing, (damn so many cores involve Slowking). Motor Drive is pretty cool so if you come in on a T-Wave from Slowking / Uxie, you're pretty much up for a sweeping condition. Nonetheless, while Electivire isn't that good, he's decent with the proper support (which he needs a lot of, perfect for C-Rank), so Mid C it is.
Hariyama is brutal, especially in Trick Room, as Guts CC is nothing to be trifled with. Priority Bullet Punch and Stone Edge make it pretty deadly as well, so this thing can punch some pretty big holes in teams. Hariyama is also a decent special wall with Force Palm, Whirlwind, and RestTalk, but eh. Anyways, solid Mid C imo.
Linoone is pretty deadly, with Belly Drum E-Speed wrecking shop. Linoone has enough coverage to just destroy everything, and Shadow Claw shits on Rotom and Golurk while Seed Bomb handles Quagsire, Poliwrath, and Rhydon. It's really frail though so it needs Memento support, but either way it's a mid C because it wrecks shop.
Primeape is an okay Pokemon, with a usable Speed tier and U-turn allowing it to be of enough use in RU. It has a decently strong Close Combat while Punishment can deal with random CMprits and SD Gallades, and that's about it. Outside of that, Primeape is pretty meh, though it's an okay Pokemon who can work.
Swoobat is pretty devastating with its pseudo Nasty Plot and Stored Power making it a force to be reckoned with, as nothing can really survive and some special attackers may struggle to get past it afterwards. It's really powerful after a boost and it can really devastate teams with ease. It's really frail though and it's rather weak before a boost, so it needs some support to work well.
Victreebel is an amazing sun sweeper, as Growth boosted Leaf Blade and Sludge Bomb really wreck the fuck outta everything. It can shit on TanKing and some other cores and nothing can really survive its onslaught. Weather Ball is really nice too. Victreebel is also resistant to common priority such as Mach Punch and Aqua jet, so despite its fraility it can live weaker moves. Outside of sun though it's pretty worthless and frail, so it needs sun support to be the devastating sweeper it is.
Low:
Gabite, Golem, Sandslash, Skuntank
Gabite has nice STAB coverage, and its decent enough to work in RU if Druddigon isn't your cup of tea. He has a nice CB set and Outrage+EQ is fine coverage making Gabite hard to wall. That said, it's got a lot of competition from Fraxure and Druddigon, making it a lower C-Ranker.
Golem is okay, as it is an effective lead who can reliably set up Stealth Rock, and guarantees that thans to Sturdy, unless it is Cinccino who is ready to ruin your day. It has fine STAB coverage too and priority in Sucker Punch, so it can work. Outclassed by Rhydon though.
Sandslash, a Pokemon who I previously attempted to nominate for E-Rank, is a decent spinner. SD+Lum Sandslash is a fine spinner who can beat Spiritomb and Rotom, while it has enough physical bulk to pull off a spin. Sandslash can also kill some opponents here and there such as Drapion and Electivire. it's not that good, but it can work as a spinner if you really need it.
Skuntank can definitely work in RU, but Absol, Drapion, and Spiritomb are simply better. Skuntank has Sucker Punch, Aftermath, and Fire Blast, but that's about it.
Those are just my initial thoughts on some of the C-Rankers.
Anyways, I think these are appropriate for the remaining B-Rankers:
Top:
Mesprit is really versatile and has a nice balance of stats to make good use out of her amazing movepool. She's a case of a Pokemon that you don't know what she's gonna do 'til it's too late. She does fucking everything, from Offensive CM to Support to CB. Healing Wish is good too and makes her a good asset to teams that need it. Nonetheless, Mesprit does her jobs extremely well and is pretty unpredictable.
Mid:
Gardevoir is pretty strong and makes for a fine special attacker. Its 125 SpA and solid movepool are pretty good, and Trace allows it to check a lot of things like Lanturn, Magneton, and a lot of others. LO Gard is strong and works really well. Also, it's a decent cleric / Wish supporter. That's about all there is to Gardy.
Roselia is one of the best bulky Spikers around, with her only competition being Qwilfish. While the spiky puffer shines on teams in need of a check to things like Escavalier, Entei, and Emboar, Roselia excels on those in need of an answer to threats such as Sceptile, Galvantula, and Omastar. Roselia is a great special wall who can take on the aforementioned threats, and she sets up Spikes very reliably. Roselia is no sitting duck either; a solid 100 Special Attack with good STAB's in Giga Drain and Sludge Bomb make her even better to take on those threats. Roselia has some other support options as well, so she can always be a good team supporter if needed. While she does have some flaws such as low Defense and a rival in Amoonguss, her perks more than outshine that, making her suitable for Mid B.
Scolipede is soooo good in this metagame, the fact that it dropped to NU is a joke and a traversity. Scolipede is an amazing Spiker, he has an awesome Speed tier that not many can match, and he can set up Spikes very reliably for offensive teams. If he gets to Sash level, a Swarm powered Megahorn is amazing an can wreck. He even gets QuakeSlide coverage, making him incredibly difficult to wall. With SD and 90 Attack, he can use that incredible coverage and be a vicious sweeper as well if you're not using him as a Spiker; he can wreck whole teams to pieces using his power, coverage, and speed. He's got a bit of competition, but Scolipede has enough perks and he's so damn solid he is good in Mid B.
Seismitoad is an excellent rain sweeper, and he has enough power to leave his mark. While he doesn't have Ice Beam, Sludge Wave and Earth Power provide solid coverage to get at a lot of things. He has an immunity to Electric, so he makes a great asset to rain teams to patch up the Electric weakness they often have. He's also a usable defensive pivot with Water Absorb, and is a decent user of SR, so that's a plus. Pretty solid Mid B imo.
Low:
I think
Magneton is good enough for a low B imo. He's a solid Steel trapper and SpecsMagneton is a solid hard hitter who not only reliably traps Steels, but also checks Absol, Kabutops, Feraligatr, and more. He has a great defensive typing so he can take a couple of hits, and ANALYTIC is a cool niche as well. SpD Magneton is cool as well, as is Scarf, but overall I think Magneton is good enough to get into Low B imo.
Please move Dusknoir to E-Rank; it can't even do its job well and it has no reason to be used over any Ghost-type available in the tier.