Feeling like Chansey needs to get suspect tested ngl, it's pretty nuts. I say this as someone who has used/is using Chansey and seen the annoying crap it can do.
My main argument is that Chansey is unpredictable and hard to build around; it's got plenty of crevos that it can utilize and abuse thanks to its absurd 250 base HP and 105 SpDef. The main sets I see are Garganacl and Hatterene (the latter of which I use), but I've also seen Skeledirge and Gardevoir at some point, and they're all worth addressing. The tools given to it in Gen 9 are overwhelming, to say the least.
Chansey/Hatterene
Completely shuts down Phys Attackers who would otherwise beat up Chansey easily. Fairy type does wonders for Normal type, granting it another immunity, as well as a neutrality to Fighting type. It has access to Charm, the arguably only reason why Chansey is as good as it is. Magic Bounce, thanks to Hatterene, makes it easy for Chansey to swap in and mirror a status move or shut down common mons like Dunsparce/Clodsire, most Primeape crevos, etc.
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I did this battle as I'm making this post, and I couldn't agree more with the comment they made at the end regarding to Chansey.
https://replay.pokemonshowdown.com/gen9crossevolution-1849374739
Chansey/Garganacl
Ok, take Chansey, but what if it had Purifying Salt? On paper, this stat spread is nothing to call home about. Garganacl is annoying before hand; sure, but like, it should be manageable enough... right? Well, until you make the realization that with Garganacl, you get Salt Cure too. Steel and Water types beware! I hate the uncommon-yet-dreadful concept that it could run both Soft-Boiled AND Recover on the same team if it wanted to. On top of that, Chansey/Garganacl can set rocks and sit on Ghost types who think they could break through it. There's a lot of options for giving the bulky egg a bulky evolution.
I
had replays for this set, but I forgot to upload the game. I'm sure anyone's who has faced a Gargsey can vouch for my distaste for this set, or even if you use one yourself.
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Chansey/Skeledirge
Normal/Ghost, Unaware, and Will-o-Wisp. Need I say more? ...okay fine.
I've never used one personally, but I've heard plenty of how annoying it is to face; and it's not hard to see why. Skeledirge is a goated crevo for Chansey; giving it Hex to pair with Will-o-Wisp, as well as Torch Song to boost its and Hex damage. Normal/Ghost is a great combo with 1 weakness; Dark. It has 3 invaluable immunities to this meta; Normal, Ghost, and Fighting.
I would also like to preface that with the concept of unpredictability again; Especially with the aforementioned evos, Chansey could either be immune to Status/Toxic, reflect your Status/Toxic/Will-o-Wisp, be immune to Normal, Ghost, Fighting, or Dragon (Hatterene); etc.
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Chansey/Gardevoir
Chansey with Trace can prove to be menacing; you can utilize any of the opponent's abilities that are bound to be useful thanks to this metagame, such as Unaware, Regenerator, or any of the aforementioned abilities. You could be very sadistic and run Synchronize- wait no one does that? Oh, okay. Well, Gardevoir's crevolution gives Chansey a boost to it's SpDef (as if it
needed that). It also gives Chansey the signature move of it's line in Gen 9; Teleport. Y'know, that move that got buffed in Gen 8 to give a free switch with low priority; yeah that move. Now it can be an effective Wish Passer, deal out Will-o-Wisp, etc. Seriously, there's a lot Gardevoir crevos can do; it's nuts. Again, no experience using this set, but it's definitely good.
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Other/Closing Thoughts
Some other crevos I considered to mention (but I didn't have enough info to talk about it) are the following, with small comments with notable traits:
Also these don't have to be "good" sets, they just have to be somewhat usable in my opinion.
Chansey/Tsareena (Priority Wall + Rapid Spin)
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Chansey/Dragonite (Ground Immunity + Multiscale)
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I am noncensical though, hence the username, but I do feel like this is worth addressing. There's a few mons that need discussion IMO and this is one of them of the bunch.
Anyway, that's all. See ya.