Yay, been so hyped for this format for months and now it's finally here. Some stuff I wanna talk about...
Potential Cores
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This was one of the most hyped cores so far, and while it is a strong one I don't think it's as OP as we originally thought. The user 2 posts above this summarized why, I don't really have anything else to add, and it's still a strong core but not as broken as we first thought.
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This one I feel lives up to the hype more, as you get a lot of Speed, bulk and power, they cover each other's weaknesses beautifully and there aren't many Pokemon that reliably check them both. ShadowRex in and of itself is in a worse place this meta though, since Wide Guard is much easier to fit on your team. But if there's anything ghost horse has taught us, it's that even if you're well prepared for it on paper, it can still destroy you easily.
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A more offensive, aggressive version of the above core. Gives you the option of playing Sun.
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A more balanced take on the above core, I've personally laddered with this and I can vouch for its effectiveness. With Koraidon bringing the Speed and power and Lunala on support. Agree with the above poster, Lunala loves double restricted as its jack-of-all-trades style really fits in well here.
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Hot take, this core is better than Icerex+Miraidon. Groudon's typing lets Miraidon run Tera Electric Discharge for devastating spread damage, and Rillaboom doesn't want to switch in and risk eating a 120 BP super effective sun boosted Heat Crash. Groudon also loves being paired with Miraidon's usual entourage: Whimsicott gives it Tailwind, Light Screen and resets Sun, Ogerpon-H hits like an absolute truck in the sun, Groudon can also abuse Farigaraf's Trick Room (it can even set Gravity to fix Precipice Blade's accuracy and let it hit airborne targets). Swords Dance Groudon also likes Iron Hands's Fake Out (don't run Bulk Up Groudon, it sucks).
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The Swordfish core might be slightly neutered going into SV, but it's not to be underestimated. Personally I think you're better off going for Hydro Pump over Origin Pulse due to the popularity of Wide Guard Zamazenta, but the two cover each other's checks really well - Zacian destroys non Tera Koraidon, Raging Bolt, Rillaboom and Ogerpon-W, while Kyogre destroys the Fire types it hates. I think Bulky CM Kyogre isn't very good now, and Scarf has fallen off this gen (although it wasn't great last gen either tbf), but 3 attacks and Assault Vest are still strong.
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I don't like this core at all. I don't think Terapagos is very good in this meta (I loved using it back in reg G) because to get the best from it you have to dedicate your tera slot and remove all weather and terrains. So you have to pair it with stuff that doesn't need weather, terrain or tera, and nearly all other restricted Pokemon like to have at least 2 of those. Wide Guard also stops both of them using their best moves. Bulky CM Pagos is much worse in this format with the faster pacing.
Some cool niche mons
While it has worse stats than Grimmsnarl, Tornadus and Whimsicott, Sableye makes up for this by being naturally immune to Fake Out, letting you run Covert Cloak elsewhere, and having moves those 3 wish they had, like Gravity, Will-o-Wisp and Quash. Groudon and Kyogre both love having the goblin around, since not only can it set Gravity to remedy the accuracy of their STABs, it can also stop Wide Guards with Taunt, reset their weathers, and provide Speed control.
Another Prankster support mon, Thundurus excels in debuffing. Prankster Thunder Wave is always nice, while it has Taunt to stop support and Eerie Impulse to weaken special attackers. Pairs nicely with Koraidon, since it can reset sun, check Water types, and provide Speed control
It is a flea market Zamazenta, but if you have both your restricted slots already filled it's a solid choice. Bastiodon gives you a Wide Guard user, an Icerex check (and by that I mean it stops it spamming Glacial Lance which is frankly half the battle), and with Iron Defense it can even be a nice lategame wincon in its own right.
Araquanid has a serious who's who of Pokemon it checks. Koraidon, Icerex, Zamazenta, ShadowRex, Terapagos, Rapid Strike Urshifu...and despite that low Attack stat, Water Bubble still lets it hit hard with its Water STABs.
Now, let's say you want a bulky redirector, but Amoonguss just doesn't fit on your team because you're also running Miraidon. Well, while it can't put stuff to sleep like that irritating mushroom (and again you're running Miraidon), Sinistcha can set Trick Room, heal its teammates with both its ability and Life Dew, and even set up Trick Room. Dont you cancel out trick by using it twice
I feel like Zacian has a really nice niche rn because it can ohko Mirai in and out of tera and it has absurd coverage and is pretty fast in this slightly slower than usually metagame. Also, it can ohko grimm and whims which is very useful. I could go on and on why Zacian is good but it would take forever so I'ma stop here. Tell me if u agree or disagree.