It is my opinion that Chansey should be banned from RBY OU/1U.
Here's every 1U Pokémon in the game, listed. I'll list Chansey's matchups in a fresh 1v1. Assume that Chansey is using the standard Softboiled/Thunderbolt/Thunder Wave/Ice Beam set.
Chansey - 50/50 decided entirely by RNG, notoriously unhealthy for the metagame by being decidedly random.
Exeggutor - Sleep beats everything, otherwise, Chansey wins. Exeggutor can boom to trade at best.
Snorlax - Chansey cannot get paralyzed and thus wins vs. the standard 4 attacks Snorlax by virtue of Thunder Wave + Softboiled + the 5HKO from Thunderbolt. Rest sets are a wash, Ice Beam allows you to freeze to win.
Tauros - Thunder Wave and then Chansey 4HKOs. Tauros can fish for critical hits but even if he gets two straight BSlam critical hits it'll be gimped. HBeam crit can always bail Tauros out of course.
Alakazam - almost a wash. Neither side wants to attack too much so that it can conserve PPs. Psychic can force our Chansey though and Seismic Toss adds additional attacking PP, so let's call this a win for Alakazam. Seismic Toss Chansey is less troubled by Alakazam and can win with some timely FPs, making it more of a wash.
Starmie - Starmie loses handily.
Lapras - Sing and thus Sleep beats everything, otherwise Chansey wins. Thunderbolt 3HKOs and you're not likely to freeze before that, and because you're a Water type you can't restloop.
Slowbro - Thunderbolt and Thunder Wave are solid threats. Slowbro needs to dodge critical hits and untimely FPs for some 6-7 turns, but wins beyond that. I don't know the math but it's quite even I'd guess, with Chansey being favored?
Zapdos - again, I don't know the math, but I know that I'm not sad if I can paralyze Zapdos and then switch out Chansey safely. Even if not, Chansey has good tools to combat Zapdos with Thunder Wave and Ice Beam. Reflect SToss set makes this even easier.
Gengar - Sleep beats everything, but Explosion doesn't even OHKO with a max roll and otherwise Chansey just slowly wins this matchup.
Golem - Ice Beam 2HKOs and Golem has to boom. Earthquake isn't even a likely 2HKO.
Jolteon - high variance matchup. I don't know the math but if Jolteon is reasonably lucky with DKick/FPs it should be won by Jolteon 75% of the time or so. You pay a steep price though by getting paralyzed.
Jynx - can technically Restloop Thunderbolts, but I've tried to do that too many times only to get an untimely critical hit in my face. Still not bad for Jynx overall.
Cloyster - Clamp/Wrap beats everything slower. A miss means Cloyster's a goner though, TWave and TBolt both cripple Cloyster hardcore. Do ya feel lucky, punk? Explosion can trade.
Dragonite - See above.
The trend in the above list is that there's nothing that clearly beats Chansey and apart from Alakazam, everything that actually does loathes taking Thunder Wave. This is a major part of RBY as it is currently shaped - just the threat of Thunder Wave is enough to make a lot of Pokemon that usually beats Chansey switch out, as their one-turn damage output is just too low.
If you change Chansey to a Seismic Toss/Reflect/Thunder Wave/Softboiled set, you defeat everything relying on physical attacks even easier, at the cost of not defeating Starmie/Rest Lapras(???)/opposing unparalyzed Chanseys as easily. These two sets combined makes Chansey undefeatable "in the teambuilder".
If you accept tanking paralysis with Chansey, you lose versus physical attackers, but they still loathe Thunder Wave so you can cripple them and switch out if need be.
The matter of fact is that Chansey has literally no matchups where you can prepare in the teambuilder and feel like you've always got a way to wear down Chansey, unless you're going to Explode - but all booms that target Chansey are telegraphed. Out of Gengar, Exeggutor, Golem and Cloyster, the only one you'd switch in on Chansey is Exeggutor, and even then its accepting that you're getting 3HKOd. Reflect Chansey also makes it a moot point to explode on it to begin with. A well played Chansey always goes at least one for one as it spreads paralysis, chip damage and even if it in the end takes a boom and dies, it still was the biggest hindrance to your opponents team up until that point.
Meanwhile, Chansey has a distinctively hindering effect on RBY's metagame. Its existance holds back a plethora of Pokémon on the special side and even some on the physical side, as it is bulky to the point of ridiculousness and has no exploitable weaknesses. You always need a combination of Pokémon to beat it and it has no viable hard counters, only temporary stops that makes it switch out for some time.
If Pokémon Perfect is truly serious about testing everything to create healthy metagames, then the assumption that Chansey is healthy in RBY 1U should be challenged more seriously than it currently is.
If I am not tired tomorrow (which I am now), I might add something regarding Reflect SToss Chansey, but I think you've all seen that enough to know how impactful and mindblowingly frustrating/boring it is to face.
ban chansey and make rby fun again.
Here's every 1U Pokémon in the game, listed. I'll list Chansey's matchups in a fresh 1v1. Assume that Chansey is using the standard Softboiled/Thunderbolt/Thunder Wave/Ice Beam set.
Chansey - 50/50 decided entirely by RNG, notoriously unhealthy for the metagame by being decidedly random.
Exeggutor - Sleep beats everything, otherwise, Chansey wins. Exeggutor can boom to trade at best.
Snorlax - Chansey cannot get paralyzed and thus wins vs. the standard 4 attacks Snorlax by virtue of Thunder Wave + Softboiled + the 5HKO from Thunderbolt. Rest sets are a wash, Ice Beam allows you to freeze to win.
Tauros - Thunder Wave and then Chansey 4HKOs. Tauros can fish for critical hits but even if he gets two straight BSlam critical hits it'll be gimped. HBeam crit can always bail Tauros out of course.
Alakazam - almost a wash. Neither side wants to attack too much so that it can conserve PPs. Psychic can force our Chansey though and Seismic Toss adds additional attacking PP, so let's call this a win for Alakazam. Seismic Toss Chansey is less troubled by Alakazam and can win with some timely FPs, making it more of a wash.
Starmie - Starmie loses handily.
Lapras - Sing and thus Sleep beats everything, otherwise Chansey wins. Thunderbolt 3HKOs and you're not likely to freeze before that, and because you're a Water type you can't restloop.
Slowbro - Thunderbolt and Thunder Wave are solid threats. Slowbro needs to dodge critical hits and untimely FPs for some 6-7 turns, but wins beyond that. I don't know the math but it's quite even I'd guess, with Chansey being favored?
Zapdos - again, I don't know the math, but I know that I'm not sad if I can paralyze Zapdos and then switch out Chansey safely. Even if not, Chansey has good tools to combat Zapdos with Thunder Wave and Ice Beam. Reflect SToss set makes this even easier.
Gengar - Sleep beats everything, but Explosion doesn't even OHKO with a max roll and otherwise Chansey just slowly wins this matchup.
Golem - Ice Beam 2HKOs and Golem has to boom. Earthquake isn't even a likely 2HKO.
Jolteon - high variance matchup. I don't know the math but if Jolteon is reasonably lucky with DKick/FPs it should be won by Jolteon 75% of the time or so. You pay a steep price though by getting paralyzed.
Jynx - can technically Restloop Thunderbolts, but I've tried to do that too many times only to get an untimely critical hit in my face. Still not bad for Jynx overall.
Cloyster - Clamp/Wrap beats everything slower. A miss means Cloyster's a goner though, TWave and TBolt both cripple Cloyster hardcore. Do ya feel lucky, punk? Explosion can trade.
Dragonite - See above.
The trend in the above list is that there's nothing that clearly beats Chansey and apart from Alakazam, everything that actually does loathes taking Thunder Wave. This is a major part of RBY as it is currently shaped - just the threat of Thunder Wave is enough to make a lot of Pokemon that usually beats Chansey switch out, as their one-turn damage output is just too low.
If you change Chansey to a Seismic Toss/Reflect/Thunder Wave/Softboiled set, you defeat everything relying on physical attacks even easier, at the cost of not defeating Starmie/Rest Lapras(???)/opposing unparalyzed Chanseys as easily. These two sets combined makes Chansey undefeatable "in the teambuilder".
If you accept tanking paralysis with Chansey, you lose versus physical attackers, but they still loathe Thunder Wave so you can cripple them and switch out if need be.
The matter of fact is that Chansey has literally no matchups where you can prepare in the teambuilder and feel like you've always got a way to wear down Chansey, unless you're going to Explode - but all booms that target Chansey are telegraphed. Out of Gengar, Exeggutor, Golem and Cloyster, the only one you'd switch in on Chansey is Exeggutor, and even then its accepting that you're getting 3HKOd. Reflect Chansey also makes it a moot point to explode on it to begin with. A well played Chansey always goes at least one for one as it spreads paralysis, chip damage and even if it in the end takes a boom and dies, it still was the biggest hindrance to your opponents team up until that point.
Meanwhile, Chansey has a distinctively hindering effect on RBY's metagame. Its existance holds back a plethora of Pokémon on the special side and even some on the physical side, as it is bulky to the point of ridiculousness and has no exploitable weaknesses. You always need a combination of Pokémon to beat it and it has no viable hard counters, only temporary stops that makes it switch out for some time.
If Pokémon Perfect is truly serious about testing everything to create healthy metagames, then the assumption that Chansey is healthy in RBY 1U should be challenged more seriously than it currently is.
If I am not tired tomorrow (which I am now), I might add something regarding Reflect SToss Chansey, but I think you've all seen that enough to know how impactful and mindblowingly frustrating/boring it is to face.
ban chansey and make rby fun again.