Trading Snorlax for Chansey (Selfdestruct) often results in the owner of Snorlax losing the game. A whole lot of RBY games and metagame trends revolve around Snorlax's moveset and this tells a lot.
However Snorlax needs Rest to switch into opposing Snorlax and to get rid of paralysis, at that point something is going to resist its attacks. Not only that, Rest relies largely on your opponent using Chansey or at least on wide paralysis support. I wouldn't be surprised to see Amnesialax becoming more popular, even with greedy movesets that don't include Rest.
Starmie (and possibly Alakazam) are now looking even better than Chansey at what it used to do (spreading paralysis and stalling), so is it time for Chansey to leave the S club? I still like the idea of a sleeper with a recovery move and Ice Beam, but we'll find out.
Starmie is the most dangerous pokemon in the game after Chansey is gone, it can tank Body Slams and pressure Snorlax with Surf. Thunderbolt Chansey isn't common.
Surf Starmie can beat Alakazam too. Unfortunately, Starmie + Chansey means you won't be able to pressure Chansey all that much, especially if your Snorlax has a moveset that your opponent can wall; this gets even worse if you are using Exeggutor too.
Starmie is a better lead than Alakazam because it has access to ice moves and better defense, it also has the best win rate among leads.
If we look at stats, back Starmie isn't doing well but I think that Starmie is just getting splashed too often on too many teams (replacing Exeggutor),
including bad builds. Or at very least builds that aren't well positioned right now.
Starmie isn't mandatory because it doesn't fit perfectly each and every single team, but every time you'll decide not to use it and you'll lose the game it will leave yourself questioning your decisions.
This is thanks to the good old tools: Sleep Powder, Psychic, Explosion.
Alakazam is just is a bad attacker (which is why we're not spamming Reflect) which you can't build a plan around, it's not going to sweep and most of the games where it creates some trouble it was thanks to RNG and/or setup. In both cases there are way easier to set up and more devastating threats available.
Sure, it can shine under certain circumstances, just like against CloyJolt builds (which aren't common right now) but that won't do.
Zam looks good in teams that don't use Chansey (or use it as a sleep fodder), or in S3 teams that don't want to add weaknesses to Zapdos (like lead, S3, Alakazam, Cloyster/Slowbro).
It still is a good lead but I don't like the fact that it can't spam Ice moves against Sing Chansey so you have to resort to going Egg turn 1 sometimes.
When it's in the back, Zam isn't as versatile as Starmie and it is demolished by Slowbro or by physical attacks.
Not only that, back Starmie has Surf and that makes para'd Alakazam vulnerable. Rhydon usually is a better switch into Gengar.
All in all it has flaws but you can live with them once you have them figured.
Snorlax's most dominating moves are Hyper Beam and Selfdestruct, which Rhydon takes no problem; not only that, he can answer Gengar when you are using monoLax.
Rhydon is extremely hard to switch into once Starmie is para'd and it will also 3hko Slowbro more often than not. Its critical hits are almost as good as Drill Horn, dealing with favourite targets monolax and Reflect Chansey.
OHKO on Jynx, Gengar Jolteon, Articuno, Moltres, can't be trolled by unstatused Chansey or Porygon (and Cloyster to some extent).
Rhydon doesn't beat RestLax and it makes your team weak to Jynx's lucky openings, Psychics and BeamBolt/Cloyster.
So, fitting Rhydon has always been a problem to me. In Invitational 3 Rhydon was used in almost 20 different builds! Starmie/Rhydon/S4 is still doing well (possibly with Sing Chansey as the lead/sleep fodder).
Jynx/Starmie/Rhydon beats Zapdos consistently and has a 50% win rate against teams without it.
Starmie/Rhydon/Cloyster/Sing Chansey performed well even though the sample was small; I don't like that team though. It's just too weak to Starmie, and if Chansey gets frozen it's in trouble.
I can't really fault you for putting Cloyster as your number 7 and Rhydon as number 8 even though I'm impressed with Rhydon (when things don't go extremely wrong) and you have to put some red lights for Zapdos sometimes.
Cloyster can be used in any archetype (classic stall or aggressive explosions/chanseyless) but not on any team. I like it as a stand-alone (lead + S4).
There are certain matchups where your opponent will have serious troubles getting rid of it and you have the emergency (Explosion) button for the rest.
Clamp takes out Substitutes for free; it works well against Reflect Chansey and Slowbro. Cloyster punishes rhydon+Egg cores and it can also switch into para'd Jynx. It also stops degenerated stuff like MonoIceLax. It's definitely a staple due to its ability to be both a defensive pivot and a late game threat. Thunderbolt Tauros and HB Snorlax are being used a lot already though.
These are the pokemon that Cloyster is meant to counter and they learn moves that can turn the tide against it.
Victreebel is nowhere to be seen, so is Amnesialax. On top of that, Starmie's CH Thunderbolt doesn't OHKO so you can somewhat plan things including a worst case scenario that doesn't fully suck (the do nothing scenario).
Thunderbolt Tauros, meant for Cloyster, is the only bad note. Reflect Slowbro is so much better against Exeggutor and Snorlax, and it can 6-0 whole teams. Thunder Wave is still the more versatile option, useful after bad starts and it pairs so well with Amnesia (speed quartering).
Substitute makes sense too.
Rhydon is doing great and some teams have just one member who is weak to Zapdos' attacks. Zapdos is comeback material, sure. It comes with a price though.
Picking Slowbro over Zapdos, or simply putting Zapdos at the bottom of the top 10, is quite controversial but I actually used Slowbro just as much or slightly more often than Zapdos so at least you know I genuinely believe it.
It's the lead that will give you the turn 1 sleep you want so bad, even against anti-leads. Everything still comes down to whether you are able to use it as a sleep fodder or not.
Gengar is a bore for ReflectHB Snorlax and Thunderbolt Tauros (that you really want to have when facing Cloyster).
I've never been a fan of Gengar but I get the idea that it can get more value with proper building and planning, I'm quite positive it still has major flaws though.
Add Zapdos' performances (20% usage and 40%ish win rate) and it's easy to at least guess that Jolteon's place in OU is narrow right now.
Jolteon could be used as a lead, considering it's actually the only catch-all anti-lead: it can beat (Sing) Chansey or discourage it from its plan, and it obviously does great
against each and every one of Starmie, Jynx and Gengar. Will it be consistent enough, though? The fact that it has no room for both Rest and Pin Missile hurts.
Victreebel still gives you a different angle of attack, so I wouldn't rule it out completely and forever.
Psychic and Ice Beam is still great and it's ok against pretty much everything even though a bit too easy on Jynx.
It's quite frail but it is the only anti-lead that can pressure everything; unfortunately it needs Pin Missile to hit Exeggutor but you want Rest too.
I think RBY OU is going to keep changing even though you can still show up with certain classic teams and be ok. Same pokemon, different movesets.