Since nobody in here is making a viability ranking, let me make my own! This is not complete!
S Tier
None yet, regenerator is close to this but not quite.
A Tier (ah yes, definitely a tier)
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Amazing special walls, I put Chansey above Blissey as it can take hits even better with Eviolite, but Blissey can hold items besides Eviolite.
+
A great stallbreaker, it has the options to break through Dondozo, Alomomola, and many Regenerator walls. The only problem is how do you deal with curse dondozo?
+
The fluffy ability can help in some matchups against Gapdos as well as Guts and Tough Claws variations of Great Tusk. It has Roost to heal itself and Defog to remove hazards. Overall, pretty good!
B Tier
+
I may be biased but I think this is a good breaker to stall, as 6 of the common regenerator cores are weak to at least one STAB, and this can have sheer power to break through Dondozo and Alomomola.
+
Fluffy can make a pokemon great, the only thing it needs is Wish support and you have a long time staller!
+
A guts sweeper can be good especially with rapid spin to remove hazards and improve speed. It can easily break through many pokemon such as Goodra-Hisui, Ting-Lu, as well as other Regenerator cores, and Unaware doesn't stop Guts!
C Tier
+
This is able to set up rocks, taunt, and explode on a foe. With no Gholdengo Taunt can't be blocked nor explosion! Woo! Nothing else to say.
D Tier
+
The only benefit this has to Ursaluna is that it also has Flash fire to not be able to be hurt by fire, but only use this if you already used Ursaluna as a donor.
E Tier
S Tier
None yet, regenerator is close to this but not quite.
A Tier (ah yes, definitely a tier)
Regenerator is a very good ability as it helps glue teams together, but it is not oppressive due to the presence of offensive wallbreakers such as SFLO Roaring Moon or special sweepers such as SFLO Gengar. Here is a mini-tier list of all good Regenerator Pokemon!
High Tier
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Medium Tier
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Low Tier
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High Tier
+ | + | +|+|+Medium Tier
+/|+|/+Low Tier
+|+Unaware is a pretty good way to nullify setup sweepers, otherwise your Regenerator mons can get overrun with setup sweepers and wallbreakers.
High Tier
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+
Medium Tier
+
Low Tier
None yet!
High Tier
+|+Medium Tier
+Low Tier
None yet!
+|+Amazing special walls, I put Chansey above Blissey as it can take hits even better with Eviolite, but Blissey can hold items besides Eviolite.
+A great stallbreaker, it has the options to break through Dondozo, Alomomola, and many Regenerator walls. The only problem is how do you deal with curse dondozo?
+The fluffy ability can help in some matchups against Gapdos as well as Guts and Tough Claws variations of Great Tusk. It has Roost to heal itself and Defog to remove hazards. Overall, pretty good!
B Tier
+I may be biased but I think this is a good breaker to stall, as 6 of the common regenerator cores are weak to at least one STAB, and this can have sheer power to break through Dondozo and Alomomola.
+Fluffy can make a pokemon great, the only thing it needs is Wish support and you have a long time staller!
+A guts sweeper can be good especially with rapid spin to remove hazards and improve speed. It can easily break through many pokemon such as Goodra-Hisui, Ting-Lu, as well as other Regenerator cores, and Unaware doesn't stop Guts!
C Tier
+This is able to set up rocks, taunt, and explode on a foe. With no Gholdengo Taunt can't be blocked nor explosion! Woo! Nothing else to say.
D Tier
+The only benefit this has to Ursaluna is that it also has Flash fire to not be able to be hurt by fire, but only use this if you already used Ursaluna as a donor.
E Tier
would be a great place to start. Currently, most regenerator donators don't offer enough stats to be worth using at all, but implementing Toxapex both gives regenerator mons a good statline as well as introducing a new pokemon for defensive counterplay (e.g, a prankster haze Mon, or an infestation rough skin helmet mon, or a new fluffy wall).
would be a great addition. Some might be concerned that it's too bulky, but bear in mind that A)., It only has 30 more base HP than cress, a very unviable Mon for the meta (too fragile), B)., that Ghost Dragon is a very bad defensive typing (literally dies to itself) and C)., Giratina has very limited recovery and utility. For A)., keep in mind that Dugtrio and Toxicroak are only 30 HP apart both are very fragile and often get OHKO'd.
Cresselia was a brutal and often downright unfair setup sweeper. Fused with Hatterene for Magic Bounce or Skeledirge for Unaware, consistent ways to stop it, especially with Screens, are in far too short supply. Additionally, Gholdengo being unbanned adds (or returns) another tool to its arsenal.
Iron Hands was on the initial banlist for much the same reason as Ursaluna, that being its Unburden and Poison Heal sets. It is also being unbanned for the same reason as Ursaluna, we feel that without these sets it would not be an overwhelming presence in the metagame.
Iron Moth was initially banned because of its incredibly overwhelming power when fused with Hisuian Braviary, and the ways to consistently answer it being quite restrictive. However with the ban of Walking Wake making it easier to justify using specially defensive Pokemon like RegenVest Roaring Moon and Ting-Lu, we have decided that it would be worth giving it another shot.
Banned for its very difficult to answer Tail Glow sets, we decided that the defensive utility it provides with both physically and specially defensive sets would make it worth testing again.
Sneasler was a massive beneficiary of Ogerpon-Hearthflame's restrictive presence punishing its checks like Great Tusk and Corviknight. With Ogerpon-Hearthflame being banned banned however, we felt that it would no longer be overwhelming.
Banned for having too much set variety and varying answers, as well as providing Good as Gold when used as a fusion, it is being unbanned because its offensive prowess isn't too overwhelming and it brings a plethora of good defensive traits to the metagame, thus being given a second chance.
: Another Steel/Dragon joins the fight! It definitely won't fit the RegenVest role like Hoodra does courtesy of its low base SpD, but I have a feeling that Archaludon will fit into a much more offensive role than Hoodra.
: I'll keep this one short: This thing looks insane. I mean, seriously, who thought it would be a good idea to give it Sharpness? ...Oh, wait. I did.
: Hey there, little buddy! Did you get lost on your way to Ubers? Oh. You're setting up Rock Polish. Now you're setting up Calm Mind. No, you can't use Tera Starstorm on me.
literally any defensive Pokemon
(Wellspring and Cornerstone) 
and more
/ 
literally any defensive Pokemon / pivot
/ any defensive Pokemon you see fit on HStack / random offensive Pokemon on dual screens
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