S Rank
Arcanine is the best mon in the tier and it's not really close imo. This is the only mon I consider to be truly mandatory in RBY PU (see: SSJ's 100% usage). It is the best lead in the tier, and the only reason to not lead Arcanine is if you want a back Arcanine. Arcanine basically defines the early stages of a PU game, even if you don't lead it it will likely come out early to fight Fearow/Gastly/Staryu or whatever. This means the poor Seaking matchup isnt even that big a deal since Seaking is a lategame mon, so Arcanine is usually dead by the time Seaking comes out. This mon is also a high stat machine that can trade 1-for-1 with almost the entire tier.
A Ranks
Staryu at #2 I expect to be a hot take but I can explain my reasoning. This current meta is AMAZING for Staryu. One of Staryu's biggest downsides is that after taking any kind of chip damage whatsoever, it is in Fearow range and can be revenge killed by it. Due to the rise of Graveler, you are more likely to have a mon to fall back on for this inevitable Fearow revenge kill. Additionally, people are dropping Fearow more often now, meaning you may not even have to worry about this revenge kill in the first place! Graveler rising is also good for Staryu since it just ohkoes it. Staryu is also fantastic at doing what it normally does (checking seaking, nidoqueen revenge killer, fast twaver). I also believe that having at least one twaver is mandatory on a team in order to not get 6-0ed by Agility Porygon, and Staryu is the best one by a mile. Staryu does have some 4mss since you want recover to actually wall Seaking (outside of crits) but surfbolt tw recover can lose to things like Gastly, Vileplume, Dragonair etc.
Nidoqueen is still fantastic, this has been a top mon since this tier's inception. Perfect coverage, impossible to switch into, good offensive stats and bulk etc. It is also good against paralyzed Agility Porygon since it is twave immune, allowing it to spam Earthquake until Porygon eventually FPs to death. This is also the go-to lead when you want a back Arcanine. I'm not a huge fan of Lead Nidoqueen cuz of how burns/crits can fuck it up, but the discovery of Surf makes this matchup is a little better. It does have one downside though. It's very hard to justify using alongside Graveler imo. This is because having all 3 of Nidoqueen, Arcanine, and Graveler make a team very vulnerable to Seaking. Even if you lead one of Nidoqueen or Arcanine, you still have two backmons that are Seaking food. There are some specific team structures that can fit all 3 but that generally forces your other 3 mons to be Seaking hate which restricts teambuilding in a whole other way. I'll go into this more in the Graveler section but it's the biggest thing holding Nidoqueen back imo.
Fearow has obviously been hurt a ton by the rise of Graveler, but I think we may have forgotten that this mon is still really fucking good. If Graveler didn't exist this would be top 2 easily, and Graveler is not on every team, so you are still able to use broken Fearow if you dont run into the rock! Obviously if you do run into Graveler it's a lot worse off, but there are still some predictions involved (admittedly favored for the grav user). I played around with Mirror Move Fearow a bit since it can punish a Graveler that tries to use Earthquake to catch a switch. Despite Graveler rising, Fearow's ability to crit through teams and unmatched Speed tier (outside of some bad mons LOL) make it a huge threat. It is still THE nfe killer (grav is fake nfe).
Graveler had a massive glowup this PL. It can switch into Fearow and creates a HUGE momentum sink for the opposing team. Graveler can pretty much safely click EQ on Fearow to catch a switch, and pretty much nothing likes switching into STAB Earthquake. If you don't want to predict, just use Substitute. Graveler has been used so much that a lot of people have started dropping Fearow in order to not get walled by it. Even against a team without Fearow, Explosion means Graveler can still get neutral value (if it finds an opportunity to blow up, which is quite hard against a fearowless team due to its limited switchin opportunities). The main downside to Graveler is what I was talking about in the Nidoqueen section. Graveler + Nidoqueen is very Seaking weak and if you want to run both you need a very specific team structure to not fold to the fish. The problem with this is that Nidoqueen is a great mon! So either being forced to drop it or running a very specific team to fit it is not great! However, the lopsidedness of the matchup if Graveler runs into Fearow is defnitely worth it.
Seaking is a mon that I'm really torn on how to rank (just like most mons in this rank). Fearow/Graveler/Seaking can honestly be in any order imo. This is THE lategame mon. A +2 Seaking is extremely deadly and can just win the game against an unprepared team. However, your teams should be prepared to handle it. Staryu is a huge momentum sink for Seaking since it switches in then forces Seaking to either crit fish or switch out, and Staryu is very hard to switch into so this forcing out is really bad. However, if you are able to KO the checks then have an unchecked Seaking lategame, it should be able to win the game. It matches up well into a lot of mons. The rise of Graveler is also great for Seaking as well since Seaking just ohkoes it.
Gastly I think is being overhated right now, it is still a great Seaking check and has perfect coverage and explosion. However its flaws are quite apparent. Hypnosis is really unreliable and it really relies on super-effective coverage for power. It is able to hit a lot of top mons though like Nidoqueen, Fearow, Seaking, and Staryu but it notably does not do much to Arcanine or Porygon. Abra is also a really big problem from it, especially if you drop Explosion for Substitute (which is something that I don't like doing anymore due to how good of a move Explosion is). I still think it is a fantastic mon though.
Porygon has been steadily coming back into the spotlight since its massive tumble last RBYPL and it's finally a great mon again! It has a different set now, most commonly running Agility + Psychic + Recover + Thunder Wave but there are some other moves it can fit as well. The biggest problem with porygon is that it is an inherently luck-based mon due to all the crits, full paralysis, and special drops that can happen. Spamming Porygon can also be dealt with by mons like Abra (who can set up sub on mono psy) and Drowzee (altho this mon is quite bad).
Abra is the last mon who I consider to be a step above the rest of the tier. Although I was very low on this mon historically I feel like it has adapted to the current meta very well. Its glaring Fearow weakness can be patched up with Graveler, and it is great into Gastly, Porygon, and Nidoqueen. It can also set up Substitutes on Porygon and -1 Staryu which means twave isnt an instant death button like it previously was. Abra’s biggest weakness is back fires. They OHKO Abra with Hyper Beam and you can’t just slap a graveler on your team to counter them like you can with Fearow. However most Arcanines are in the lead slot while Rapidash and Magmar are less common so this isn’t a huge issue most of the time, but it is a way to exploit Abra spam. Abra also forces mindgames with back fires; Hyper Beam can be caught by a Grav switch and Slam loses to counter, meaning Blast is sometimes forced, allowing Abra to twave. This is obviously risky though cuz you can just lose your Abra if you predict wrong.
B Ranks
Machamp is a mon that I really was not expecting to do well going into this PL considering how bad it has been historically. However Graveler stocks rising are great for Machamp since Machamp is a great form of Graveler hate and is also a great partner to Graveler since Fearow is Machamp’s worst matchup by far. Fearowless teams being more common is also really good for Machamp since it is honestly a huge menace if you can’t switch Fearow in; Earthquake and Body Slam are suddenly extremely safe clicks for it if there is no opposing Fearow. Machamp is also a decent lead although you risk getting burned and generally have to play mindgames on turn 1 (EQ the arc/queen staying in or slide to catch fearow).
Vileplume is excellent into teams without Gastly and has good matchups against SurfBolt Staryu and Double-Edge Seaking. It is also the most consistent sleep user in the tier by far (gasly hypnosis is not safe or consistent at all). The only problem is if Vileplume runs into Gastly it has no real way to threaten it outside of Paralysis, which is admittedly still good since it makes Gastly Ground food. Vileplume is also good at exploiting things like unboosted Porygon. To make the most use of Vileplume you should switch out to something like Nidoqueen or Arcanine immediately after sleep to punish a stay in on predicted Stun Spore instead of trying to SD sweep since Vileplume is quite slow.
Magmar is a mon that I'm not quite sure where to rank. Its ability to force Counter mindgames with opposing fire-types is valuable, and Confuse Ray and Smokescreen can disrupt the flow of a game. I think it's a really good mon to have when your check to the opposing Seaking is your own Seaking, since getting a Smokescreen on their Seaking as you sack Magmar makes yours heavily favored to win the mirror. Psychic is also a great move since you don't have to go to a casino every time you want to attack Nidoqueen, Seaking, or Gastly. I'll talk about this more when I get to the Rapidash section but I think Magmar is defnitely better than it due to the rise of Graveler, since Magmar is a much better Graveler partner than Rapidash is.
Magneton is slightly washed but I think I would still call it PU-viable. The big elephant in the room is Graveler; it is very hard to justify fitting Magneton on a team when Graveler is everywhere right now. Despite this, Magneton still has the traits it has had in the past; being the ONLY mon in PU that has a good matchup against both Fearow and Seaking, and basically unresisted STAB outside of the grounds. However, the terrible ground matchup is REALLY holding it back currently.
Sandslash is extremely underrated rn and has a lot of positives compared to its fellow grounds. It has the best matchup against Gastly and Graveler for example so it can be a good partner to teams that are weak to either of these mons. I just haven't actually found a team to fit it. A team weak to Graveler (like Fearow + Magneton) isn't magically "fixed" by just replacing Nidoqueen with Sandslash, and my Abra spam recently has made me not have to worry so much about Gastly. I do think certian teams can defnitely make Sandslash work though.
Rapidash sucks LOL I’m pretty sure SSJ and I built ZERO teams with Rapidash this tour and I have zero regrets about that. Historically I’ve used this mon as a Fearow check since Rapidash is one of the two (2) mons that outspeeds Fearow. However, I’ve realized now that if you want a Fearow check that folds to Seaking, Graveler is right there. Graveler is a lot more consistent at checking Fearow compared to Rapidash. But what if you want to use Rapidash as a fast offensive Fire-type instead, kind of like a Fearow alternative into a high-graveler scout? Well there are two better options for that role. You can either (1) use Magmar who has a much better movepool (100% accurate psychic, cray/smokescreen to disrupt setup mons) or (2) use non-lead Arcanine so you can run Arcanine in the back. Rapidash is also tricky to use alongside Graveler, since its main entry point is revenge killing Fearow, but generally Fearow doesn't kill stuff when you have a Graveler. All of these factors combined have made it EXTREMELY difficult to justify using Rapidash on any kind of team when I feel either Graveler, Arcanine, or Magmar can fulfill its role but better.
Dragonair has similar good and bad matchups as Vileplume but Vileplume's sleep is a lot more consistent than Dragonair's Wrap shenanigans. The re-surgence of Porygon has also made nair the third-best agility user in the tier (behind Seaking and Porygon) so it is a lot harder to justify.
C Ranks
Pinsir is a mon I really wish I could have ranked higher because IN THEORY it seems like it would work really well in the current meta! Less Fearow and more grav is good for it, and less back fires is also great! Its coverage compared to Fearow is much more relavant now. However in practice I never got this mon to work for some reason. It might be builder issue or skill issue, I'm not sure. I think most of it is that its roles are better equipped for other mons. If you want a strong attacker that is weak to fearow and back fires Abra is right there, and I think Machamp is a superior anti-grav option. However this mon is definitely viable and I wish I was able to make it work.
Drowzee is mainly a matchup fish mon but if you land it into Porygon is can generally get a positive trade. If there is no Porygon it can kinda find entry on Staryu/Gastly/Abra but this can just be done by better mons (like Abra). Overall Drowzee is not that great but the value in getting the Porygon mu fish is defnitely there.
Scyther is so washed it makes Magneton look like an S-tier mon in comparison. I genuinely cannot justify bringing this to a game with all the Graveler usage. Bringing Fearow + Scyther into Graveler is a borderline unwinnable matchup. It also suffers from the same issue Rapidash does with Graveler being a more consistent Fearow check, meaning its near-unique ability to outspeed Fearow is less valuable. I think we're back in an era where Pinsir's coverage is by far the more desirable trait. Maybe when Fearow rises to NU Scyther will be in the spotlight again.
D Ranks
Arbok has 80 base Speed wrap and EQ which lets it punish Seaking/Nidoqueen/Grav stuff but you still struggle a ton against faster mons and have to rely on hitting inaccuate moves.
Omanyte doesn't even check lead fires, so ur exclusively fishing for back fires with Omanyte. You invite Waters + Gastly + Abra in for free as well and Seaking is just way better because of how much more offensively threatening it is.
Stuff I didn't rank but could maybe work:

s/o
BeatsBlack Marowak unironically can maybe kinda do something in this tier. It has the traits Sandslash has (good grav/gastly mu compared to other grounds) and it also has moves like Counter and Blizzard which Sandslash lacks.
Slowpoke could run a set of Thunder Wave + 3 Attacks I feel could maybe work on Slowpoke. I didn't rank it because I haven't used it or built with it but I think this is possibly a set that could maybe exist.