After somehow getting into RBY Cup Final, I think my opinions won,t be immediately disqualified as a madman nonsense, so I can post them. Everything is based on my experience playing both ladder and the actual RBY Cup sets. Originally after getting eliminated in the rest of Classic Cups I planned to use some not very serious teams in Round 3, but it ended up being the year in which I played more RBY battles, most of the time in high ladder. The ammount of teams I built this year (this year being the last 2 months) and didn,t delete on the same day is 35, though not all of them are equally good and I did use in earlier rounds teams built in other years that I believed to be good enough. Feel free to disagree with my opinions anyway.
S rank.
1.

Who did you expect to see here? Pretty much a mandatory Mon, only 1 team I built this year didn,t have it. Best revenge killer, hits hard, crits a lot and even a little unpredictable, since Cloyster can be smashed with Thunderbolt. A very competent lead too, since it threatens both Jynx and Gengar. As a lead, Rest is good to have on it, but the move works even outside the lead position,
Excal and
BeeOrSomething used it too, I wasn,t the only one. Nothing more to say here, Tauros is the best Mon in RBY and you shouls use it in like 95% of the teams.
2.

The other mandatory Mon. Checks every Special Mon in existance, provided something has already been Slept. Disuades brainless Thunder Wave spamming (though in reality it doesn,t like paralysis, it just hates it less than other Mons and less than all other status except Burn and Poison), spams status itself (except vs opposing Chansey). Like Tauros, I used this Mon in every team built this year except one (not the same team that lacked Tauros). Its even more versatile than the bull, since it can para, sleep, freeze and use Counter, so careful scouting is required when facing the blob. Reflect Chansey is a thing too, though I personally don,t like it. Unless frozen or slept, Chansey will do something in every battle. Unlike Tauros, I consider this Mon to be a very poor lead, since its abused by every Sleep lead, even Exeggutor (who I consider to be a bad lead too). Therefore, Chansey is second best Mon and Tauros 1st, but you should still use it in like 93% of your teams.
A rank.
3.

Common sense requires me to put Snorlax at 3rd position. My building and performance in the tier improved a lot after I stopped seeing Snorlax as a mandatory Pokemon in RBY and stopped automatically including it in every team I built. Out of 35 teams built, I had it in 19, which is still above 50%. In RBY Cup battles (20) I used Snorlax in 13 battles (65% usage). Of those, I won 9 (69% win rate). Of the 7 battles in which I didn,t use a Snorlax, I won 4 (57% win rate). Really small sample, but it does look that using Snorlax is still better than not using it. However, I still believe that some teams can be good without Snorlax and be worse if a Mon is replaced by Snorlax (for example, the one I used in G1 of Semis, which is my favorite team out of the 35 I built this year:
https://replay.pokemonshowdown.com/smogtours-gen1ou-790317 ). Just like any Mon, Snorlax enjoys paralysis and is one of the biggest progress maker of the Tier, especially against Chansey. Main issue is, Snorlax often has to do too many things without having the typing or the bulk to resist things: unlike Exeggutor, it doesn,t resist Ground, Water, Grass, Electric and Psychic, unlike Rhydon and Gengar it doesn,t resist Normal, unlike Starmie and Cloyster it doesn,t resist Ice, unlike Jolteon it doesn,t resist either Zapdos STAB. So, in practice, its not a very reliable check and you can,t brainlessly swap it into any Move like you can do with Chansey for Special moves or with Cloyster/Slowbro for physical ones. Normal type is the best one offensively, but for Snorlax, the only benefit it provides defensively is not being para'd by Body Slam. There will be battles in which Snorlax will be critted or haxed and won,t have time to execute more than 1 or 2 moves. Fortunately, those 1 or 2 moves are often enough to make a difference, since Snorlax hits damn hard. Out of the 3 main Snorlax sets (Reflect, physical 4 Attacks and Amnesia) I like purely physical Lax the most. Its the one that abuses paralysis and forces progress the best. Reflectlax is often good for long games, but has the problem of being walled by... opposing Reflect Lax. I don,t like and am not good at handling this situation, so when possible, I avoid it. Meanwhile, Amnesia Lax is a big match-up fish and requires the most support. Snorlax is a versatile threat with an enormous movepool, so its still should be considered the 3rd best Mon and one that should be included on the majority of the teams, just not all of them. Btw, props to
Isza for trying the lead Lax, I don,t believe its a good lead, but it did wreck all my gameplan when I faced it, since on ladder I haven,t seen it more than once or twice (and I played a lot of ladder games).
4:

On days I am drunk, I think this is the 3rd best Mon instead of Snorlax. I certainly use it more often than Snorlax (21 teams compared to Snorlax's 19). Sleep is huge, Boom is huge, resisting Water, Grass, Electric, Ground and Psychic at the same time is huge. Outspeeding Chansey, Lax and Rhydon (to whom its the best check) is great. Exeggutor can do everything for a team, though it has the drawback of often trading with itself. Due to outspeeding 2 of the 3 Mons above it in the VR, Exeggutor forces mind games: will this Egg Boom on my Chansey, use Stun Spore (or Mega Drain) on the Starmie that I am about to switch into it or take 30% (with a 30% Special drop and not insignificant crit chance) from my Snorlax? Egg can check Gengar and Alakazam too and forces Starmie to use Blizzard if it doesn,t want to be walled. Like Snorlax, Egg often is tasked by way too many tasks, but like Snorlax, every free turn it has it can make some progress. At the beginning of the RBY Cup, I liked Mega Drain as the 4th Egg move, but at the end of it, i alternated it with Stun Spore and Rest. About Rest, it makes Exeggutor much more reliable at beating or sleeping Alakazam. Without Rest, many battles can have Egg paralyzed, missing every Sleep Powder and or not moving vs Alakazam, with Rest the odds of beating Zam or forcing it out increase a lot. Last word about Egg, for me its a bad lead due to being outsped by Jynx (free Blizzard), Starmie (free Blizzard with high crit chance), Gengar (free Hypnosis or Boom with high crit chance) and Jolteon (TW + Pin Missile very often wear down Egg a lot). Its one positive match-up is against Alakazam, even Tauros lead is not comfy for Egg, since while getting Tauros slept or exploded on is great, Egg is pretty much forced to stay and can get haxed or at the very least worn down for something crazy like Rhydon or Slowbro (or even offensive Snorlax) to abuse that.
5:

On days I am depressed, I think current VR is right and Starmie is above Exeggutor. Been liking the Star more than I used to in previous years. 32 Recover PPs, high Speed and colourful offensive movepool. Only Mon faster than Tauros that somewhat can switch into it and force it out without dying. What are Starmie problems? 4 slot syndrome is main one. It absolutely needs Recover and almost always needs Thunder Wave too. After that, it has to choose between STABs, Blizzard and Thunderbolt. Without Psychic, paralyzed Chansey walls Starmie forever. Without Thunderbolt, opposing Starmie walls Starmie forever (and Slowbro set-ups on it). Without Blizzard, Egg walls Starmie forever. Surf is the least useful offensive move, but still nice to have vs Jynx and to reliably OHKO Rhydon (Blizzard without crit doesn,t). Starmie often is forced to be slept by Jynx, though it at least forces paralysis on it (if Chansey doesn,t come instead). Speaking about paralysis, Starmie dislikes it less than other fast Mons (and it allows it to forever wall Jynx, which is very nice), but still doesn,t like it. Starmie does however make Cloyster and some low ranked Mons (see:Fire types) a liability. It also forces out lead Gengar due to high crit chance of Psychic. Overall, a top tier Mon that is especially good in hands of good players (not so much in mine, but I did improve at using it). My favorite move combination is TW, Recover, Psychic and Blizzard. Starmie is the second best lead and notable for kind of beating the best one.
6:

The final battle vs Excal made me reconsider my opinion about Alakazam, if I won, I would be putting it much lower due to personal failure at using it. Very frail Mon, but its very fast and with high crit rate. Unlike Starmie, it doesn,t switch into Tauros well, but does way more damage and out of all Special Attackers, its the one that most likely (aka, almost always one or one) beats Chansey, without even needing some tech (like Submission) for it. Its a decent lead too. I tried some tech on Zam (Reflect, Kinesis, Metronome, Psybeam for more PPs, Counter and Submission), but in the end, TW, Recover, Psychic and Stoss is the best set most of the time. Some Mons are very shallow for creativity, though at least Zam is a competent lead.
7:

Another Mon that is very shallow for creativity, 95% of the time you use either Rock Slide or Rest on last slot. If I have many good ways to pressure Egg and Cloyster (hi Thunderbolt Tauros) with the rest of the team, I use Rest, otherwise Rock Slide. Rhydon is (with the permission of 4 Attacks Lax... or even without the permission) THE yellow magic abusser and the Mon that makes Zapdos almost unviable. Rhydon is great into Seismic Toss Chansey and into Snorlax that don,t use coverage. Rhydon only has 1 good long term check in Egg, and even that one can be worn down over the course of a match. Rhydon punishes Hyper Beams, Booms and Zapdos usage. If Rhydon crits with EQ, opponent is doomed. Overall, I like Rhydon a lot, despite needing quite of a support. Such support is not hard to achieve though, since with every team having Sleep and every OU Mon except Cloyster having a way to paralyze, free turns are within Rhydon reach. Great Mon, outside of countering Electrics, really deserves the A rank.
B rank.
8

Best lead and the main reason Starmie is as high as it is. Can put out of comission 2 Mons at the beginning of the game (example:
https://replay.pokemonshowdown.com/smogtours-gen1ou-792543, Alakazam got to wake up, but it was too late) and absorb opposing Sleep or para (if its para, then it can switch into Exeggutor's Sleep Powder). Can wall Chansey with Rest (be ready to face the Snorlax or Tauros that will come for doing it, though) or can Counter some Body Slam. Would put Jynx even higher, but its not as good as a non-lead (unless you use lead Gengar that explodes on opposing Jynx before sending your own, of course).
9

The unviable Mons punisher. List of Mons that Gengar can switch into with low or non existant risk (provided one Mon that is not Gengar has been slept already): Venusaur, Beedrill, Pidgeot, Raticate, Golbat, Persian, Primeape, Farfetch'd, Muk, Kingler, Hitmonlee, Hitmonchan, Weezing, Seadra, Seaking, Scyther, Gyarados, Kabutops. Most of those (with the notable exception of Persian) are bad, but many are bad mainly because of Gengar. Muk could be switching into Chansey, threatening Rhydon with Mega Drain and exploding into something, but Gengar existing prevents that. For me (since I use a lot of low tier Mons), Gengar is especially dangerous as a non-lead, but as a lead its already dangerous enough. If Alakazam doesn,t crit, it will be exploded. Jynx will be exploded. Vs Starmie Gengar can try to hit an Hypnosis, same vs Egg. Hypnosis its actually an unreliable move, therefore I personally rarely run Gengar, despite fearing it a lot. Gengar punishes Booms even harder than Rhydon and forces Tauros and Snorlax to run EQ. Tauros especially would rather run Boltbeam always if it wasn,t for Gengar. Persian would also be a much better Mon if it wasn,t for Gengar.
10

I am a Jolteon believer and not only because its an Eeveelution. I genuinely think its the best Electric in the Tier, not Zapdos. This is because it actually checks Zapdos and does some damage to Rhydon with Double Kick. Insane crit rate while not being weak to Ice Beam separates Jolt from Zapdos and allows it win vs Chansey way more often. Decent defensive value for something this fast and it actually dislikes paralysis less than Tauros and Alakazam. My favorite set is TW, Tbolt, Pin Missile and Double Kick, however Rest is good on more defensive teams. Jolteon is also a decent lead with its yellow magic (also can predict a Rhydon switch-in and send Egg), though in both RBY Cup battles in which I used it as such, I lost (including the decisive one vs Excal), while every time (3 times) I used it as a non-lead, I won. Jolteon needs either Egg, Cloy or Starmie to work, preferable the former (or more than one). The payoff is big and this Mon entered a lot of my teams.
11

Hello, best Snorlax and Tauros check! This guy checks the best Mons, while exploding on Chansey and using Blizzard on Egg. Unfortunately, instead of doing that it often has to just explode on Starmie, which otherwise walls it to no end. If Starmie is asleep though (or frozen), Cloyster can do a lot. Clamp helps pivoting and wears down some would-be checks. Apart from Starmie, the other big problem Cloyster has is being totally unable to paralyze, its the only OU Pokemon that lacks TW, Body Slam or Stun Spore (even Jynx has Body Slam, though very rarely uses it).
Zapdos rank.
12:

In my experience on both ladder and especially RBY Cup, this Mon is a fraud. In RBY Cup I faced it 3 times. One of them I had both Rhydon and Jolteon, so nothing to blame on Zapdos, but in the other 2 I had neither and still won. Unlike every Mon ago, when I built my teams, at no moment I had thoughts like "This team needs better Snorlax switch-ins, so I gotta use Rhydon + Cloyster" or "Jynx really screws me, Starmie is totally needed" or "Too many Special Attackers, Chansey will wall me, Lax goes in". You don,t put randomly Zapdos into teams, you have to build a team around Zapdos, just like you have to build it around Kingler, Arbok or Hitmonlee, if you want to use any of them. Unlike those, though, you have to account for the opponent having Zapdos, which meant that Rhydon and Jolteon entered into my teams very often. Zapdos absolutely requires Exeggutor on the team, even more than Jolteon. I did lose some games on ladder vs Zapdos, but still don,t think highly on it, its way less independent Pokemon than any of the above Mons, very high risk but not so high reward Pokemon. Still should be considered OU however.
OU/UU Cutoff according to me.
C rank.
13

So, uh, Gengar. Fuck Gengar. But Gengar is far from the most common Mon and is also way more likely to be used as lead than in the back. Back Gengar are terrible news for Persian, but lead Gengar will most likely be dead by the time Persian appears. Other than the huge drawback of Gengar existing, Persian is a great Mon. It has to be used carefully, since para is terrible for it, but it does good damage to everything that is not named Gengar. Persian abuses every free turn that is given to it, especially if it predicts the opponent (Snorlax most of the time) using Rest. Persian wrecks the Reflects, outspeeds Tauros and wears Cloyster down (or even straight up kills with crit Thunderbolt) for your own Tauros, who instead of Thunderbolt now can run EQ (for guess who, Gengar!). High risk, high reward Mon.
14

Used this Mon twice during the Cup. First time it didn,t do much, despite surviving the battle, which I won. Second time it great. Then in the Finals, Excal used another one vs me and caught me using Rhydon + Golem + Slowbro on the same team. Needless to say, it was the Mon I wanted to see the least in that game. Victreebel is the best Wrapper of the Tier and punishes Rhydon + paralyzed Starmie cores unless they have Exeggutor. Like Persian, needs to be used carefully, para ruins it. Can achieve great results in some match-ups, in other it will just paralyze and be walled... by Gengar.
15

My history with Slowbro is a lot of ladder games. I rarely encountered Victreebel or Venusaur on ladder, which would have been the worst match-ups for it (and which happened in the actual important game). Despite that, it very, very, very rarely swept. Slowbro is a hax magnet. If Chansey or Starmie has Thunderbolt, it will crit before you remove them. Rhydon will switch on your Rest and 3HKO 69% of the time (without even crit, but it will crit to infuriate you even more). Snorlax will paralyze you, crit you with another Body Slam and then will use Hyper Beam to finish, if it fails, will just explode. This is not to say Slowbro is a useless Mon, it just won,t sweep by itself, regardless of the support you give to him. It will however do things. Slowbro will force many opposing Explosions (which you can predict and send Rhydon into) and paralyze many Mons (Rhydon also enjoys this). If Chansey doesn,t have Thunderbolt (or Sing, but by the time you send Slowbro, Sleep Clause will most likely be already in effect), it will be set-up fodder, if Chansey does have Tbolt and you don,t want to get crit, paralyze it and send Rhydon. Rhydon is mentioned a lot here and thats for a reason, its one of the best teammates that Slowbro can have (unless you encounter Victreebel, of course).
16

This Mon wants Blizzard, Thunderbolt, Body Slam, Hyper Beam, Confuse Ray and Rest at the same time. I didn,t say Sing, because I consider it to be bad on Lapras. Chansey can get away with Sing, it can afford to miss due to having huge special bulk and reliable recovery. Lapras doesn,t have this luxury. Still a decent Mon, since its the only Ice one that isn,t walled by Waters, but very hard to use well.
17:

My favorite legendary bird, yet I didn,t use it at all this year. However, found 2 in the finals of the Cup and it was a big threat both times. Articuno needs Starmie removed and Chansey to be down (or frozen/asleep) to start being useful, but if the situation happens, its almost unstoppable. In good match-up it does less than Zapdos, but in bad match-ups (Starmie+Chansey) it does more.
18:

Wrapper, sometimes sweeps. Who gives it problems, though? Ah, yes, Gengar. For this reason, I consider Thunder Wave to be better than Agility on Nite, this way it at least support teammates rather than relying on not missing Wrap. More flexible.
D rank
19:

Failed badly both times I used it, but Boom + STAB EQ will always be useful.
20:

Better Sword Dance sweeper than Victreebel is. Unfortunately, totally walled, by guess who, fucking Gengar. Thats without mentioning the Psychic, Flying and Ice weaknesses. Still sometimes sweeps.
21:

If you want to use 3 STAB Body Slam users but don,t want to be walled by neither Gengar (Persian), nor Rhydon (Dodrio), this is the Mon you use. Decent Counter user.
22

Would be much better if Exeggutor was a good (and therefore common) lead. Unfortunately, it isn,t. Still, not the worst Mon to risk getting slept (by Jynx or Gengar), while trying to sleep or para whatever you want. Can be a fun Meditate + Headbutt sweeper, but very hard to pull off.
23

Fight STAB is actually more important on this Mon than Amnesia, with 3 Normal Mons being the best of the Tier. Similarly, Blizzard is more important than Water STAB, due to Egg. It can get away with not running Amnesia and Water STAB at all. Hard to use Mon with many weaknesses, regardless of the set, but still the best Fight Mon due to versatility. Counter is underrated move, since it can sometimes scare Tauros and Snorlax after they use Body Slam.
24

Good SD + EQ sweeper, but you still need to account for Exeggutor and Starmie (not even speaking about Blizzard Tauros).
25

Seen some Replay on RBY Cup with this Mon, but haven,t faced it on ladder at all. Is this Mon still being used. Regardless of that, since I don,t consider Snorlax to be a mandatory Mon, Porygon gets the last spot on D rank.
Other Mons I used this year, either on ladder or in the Cup.

A worse Raichu that instead of hitting Grounds, hits Victreebel with Psychic. However, that is not important. Just like Raichu, has Submission for Chansey, but hits weaker on both sides. Learns Metronome to launch a Horn Drill to Exeggutor. F rank if it existed.

Competes with Flareon for a spot on a team, but outspeeds more things and has Submission, which is nice. F rank if it existed.

Don,t use this Mon. Not only you will get walled by Rhydon, you will also make Gengar happy. And thats not a good thing. It look way too many Gens to invent Dual Wingbeat. Unranked.

Not the worst Sword Dance user, has Slash to break Reflect and has Submission to do actual damage to Rhydon. +2 Hyper Beam is very strong. What is the problem here? Ah, yes, Gengar! E rank.

Second Gengar for people that want to use some weird shit and see Gengar exploded. However, good OU teams will easily punish this. F rank if it existed.

Best Fire Mon in the Tier. Its a Normal Mon that has Fire coverage. It needs to be used carefully, but it gets good damage done. Can be used with or wirthout Fire Spin. Killing weak Mons with Quick Attack is fun. E rank and one of the top E ranks.

The Mon that can,t be OHKOd by anything that doesn,t crit. Hits hard back and punishes EQ Less Lax a lot. Can break Chansey with Submission and good prediction, has Blizzard for Egg. What is the problem here? Not Gengar this time, but Starmie, which actually is even worse. Fortunately, Starmie is a good and common Sleep absorber, so just sleep it with another Mon and start spamming Hydro misses. E rank.

Trusted this Mon twice in the cup, lost both games. Still can get some wins with SD, but if a special Mon (Starmie, Alakazam, Chansey or Egg) crit, its over. E rank.

Would be much better if people didn,t know it learns Surf. Like in the anime, someone sends their Rhydon, but since you are the protagonist, you have spent years teaching Raichu Surf to overcome the ground weaknesses and surprise your foes. Unfortunately, in the perfect information world, things don,t work this way. Raichu can break Chansey with Submission, but Exeggutor is impossible to overcome. E rank.

Outspeeds Egg, can burn Rhydon with 30% burn Fire Blast, explodes and Waters are hit with Thunderbolt. 40% Sludge helps Wrap based teams. Wait, what is that Ghost Mon that came to wall Weezing? F rank if it existed.

Has Counter and Submission, therefore can fight vs OU Mons. Except Starmie. F rank if it existed.

SD + EQ. Except it never gets to do it and is outsped by Starmie and Tauros. F rank if it existed.

Could try to use Reflect + Rest, hits pretty hard, but less hard than Flareon (though resists physical hits better and outspeeds staff). Not having Fire Spin is bad though, even Ninetales and Rapidash are better. Unranked.

In the past I used to think this Mon was only viable on teams with 5 Ice Mons. My opinion changed since then. Dewgong resists hits much better than Jynx and unlike Articuno and Cloyster, it learns Body Slam, also resisting Water (therefore Articuno loses to Cloyster, while Dewgong forces an Explosion at worst). As a result, now I think Dewgong can be run on teams that have Lapras and Cloyster as other Ice types, needing just 3 instead of 5. It even has some minor advantages over both: its faster than Lapras and hits harder on special side than Cloyster, while having Body Slam. Headbutt is also a niche move. Still very hard to use, but not a terrible Mon on its own. F rank if it existed.

If opponent doesn,t use Gengar (fuck, not him again) and you status and wear down the psychics, it can do some real damage, both with Hi Jump Kick and Counter (of scared Normals). Decent Mon, just hard to use. E rank.

Terrible, and outside Thunderbolt fully outclassed by Alakazam. Can use Counter. You would think that it can break Chansey with Submission (with crits and prediction it can), but it actually has less Attack than Alakazam. Unranked.

I tried to use this Mon a lot of times this year, I used to believe in it in the past. But I have come to the conclussion that every Fire Mon in RBY except Flareon is bad. You can have some moves that miss, but Moltres has all 3 offensive moves with bad accuracy. With so many weakneeses, its really hard to use this bird. E rank.

Persian is better except for the ability to hit Gengar. F rank

Switches into Chansey and threatens with Boom. Just like Weezing, very fucked by Gengar. F rank.

Amnesia user, better at it than Poliwrath due to having less weaknesses. Can use Counter, since opponents will try to prevent set-up with physical moves. E rank.

Non ironically, the pure Poison that I got more ladder wins with. All due to the fact that it isn,t walled by Gengar and outspeeds a lot of staff even before using Glare. Wrap is always nice to cheese with. E rank.

The goat. Needs huge para support, but can Wrap or sweep with SD + Hyperbeam if you get that far. One of the few low ranked Mons that will ever win a RBY Cup Final battle. E rank, high E rank.

and

Ok Grounds that can hit Egg and Waters with their coverage. Too many weakneses though, need a lot of support and careful playing. E rank.

Strong and with good sinergy with Rhydon. Needs to avoid both para and Thunderbolt, however. E rank.

I consider this a better lead than Exeggutor, since unlike it, if it gets to move, it will hit 255 out of 256 times. Not factoring crits, Jynx has only 41% to OHKO with Blizzard, and that is the worst lead match-up Parasect has. If it gets slept before moving... well, they slept a Parasect, they won,t sleep your Rhydon or Chansey now. Speaking of Rhydon, Parasect can switch into EQ and resist Rock Slide to status it or damage hard with Mega Drain.
Thats it, won,t be speaking about Mons below current D rank that I haven,t used myself this year. Have a nice Eeveening.