In this post, I'd like to talk about some defensive cores in the current metagame. If you've been playing and building lately, I'm sure you've seen these everywhere.
I talked about Seismitoad in a different post, but in short, Seismitoad is one of the best Stealth Rock setters right now, being capable of checking Dracovish, Rotom-W, and Rotom-H while also being annoying to deal with in general thanks to its access to Toxic and good STAB moves. Because of this, it synergizes very well with Corviknight, which is greatly annoyed by Electric-types such as Rotom-W and Rotom-H. Besides Corviknight, not a lot of Pokemon can check Excadrill super well, so it can provide these teams with very useful utility as well, be it as a potential Bulk Up wincon, Defogger, or even both. If it's running Taunt, Corviknight is also incredibly good at shutting down opposing Corviknight, which can often be difficult to do otherwise.
Good Teammates
Clefable synergizes very well with this core. In particular, I've really liked specially defensive Clefable with these two. Seismitoad and Corviknight can struggle with Pokemon such as Dragapult and Life Orb Clefable, so a Pokemon that can check both of them, such as Clefable, is of incredible value. On top of that, Clefable supplement Seismitoad with a lot of longevity through Wish, which can make it very frustrating to actually KO Seismitoad in order to make progress.
Mandibuzz is another Pokemon that has great synergy with this core. It frees up Corviknight to run Bulk Up sets, as Mandibuzz is one of the best Defoggers in the tier, while checking Pokemon like Dragapult, Hydreigon, and Gengar, which can be hard to deal with for Seismitoad and Corviknight. On top of that, it also frees you up a little by allowing you to run Life Orb Clefable if you so wish, which is a very potent wallbreaker right now.
It's Ditto, so of course it fits.
Another notable core that I've been encountering more is this core. It's capable of covering a pretty wide variety of Pokemon, like Life Orb Clefable, Dragapult, and Dracovish. Specially defensive Hippowdon is an amazing anti-meta call right now, as it's capable of annoying a lot of common wallbreakers and can hamper progress really well. Together with Ferrothorn, they can form a very good hazard stacking core that can be really hard to break down if the opponent doesn't have any specific way of pressuring them, such as with Dracovish, Barraskewda, and Hydreigon. Although it's unable to check Hydreigon, Jellicent rounds this core out very nicely by dealing with threats like Dracovish and Barraskewda pretty well. On top of that, it alleviates some of the passivity issues that this core can run into otherwise.
Good Teammates
As I pointed out, this core is particularly weak to Hydreigon, so Mandibuzz makes for a great partner to these Pokemon, especially when run alongside another Pokemon such as Galarian Darmanitan or Dragapult.
I would also like to talk about a few Pokemon in particular.
Rotom-H is a very interesting Pokemon right now, being one of the few good checks to Life Orb Clefable, Corviknight, and Togekiss, especially when backed up by Wish support. It's quite annoying to deal with consistently, as Thunder Wave and Will-O-Wisp can cripple its checks pretty easily. Beyond Seismitoad, a lot of teams struggle to check Nasty Plot variants, and it gets many opportunities to set up against Pokemon such as Ferrothorn and Corviknight.
In my opinion, Toxapex is pretty underwhelming right now. Toxapex feels really insufficient as a defensive Water-type right now because it's not a good check to Dracovish, which you basically need your defensive Water-type to be able to check at the moment. To mitigate this issue, I've been running some Toxapex + Seismitoad balances, but honestly, you could get a lot more out of that slot than Toxapex. Another issue with Toxapex right now is that it's really easy to take advantage of with prevalent wallbreakers such as Hydreigon, Hatterene, and Clefable.