Okay I've been playing a lot and definitely have some thoughts.

The best mon in the meta and it's not particularly close. Leftovers improves its already stellar longevity to the point of absurdity. It's among the best answers to many annoying threats (Primal Kyogre, Zacian-C, Ho-Oh, Arceus-Fairy, Eternatus, Arceus-Steel) and has immense set versatility and utility. I think best set currently is the standard specially defensive set: something along the lines of maximum Special Defense and HP with Precipice Blades, Stone Edge, Will-O-Wisp, and Stealth Rock or Spikes—though it has a lot of variety when it comes to moves with options like Rock Tomb, Dragon Claw, Lava Plume, Overheat, Protect, or Smack Down. I don't think all-out offensive sets are too great with the loss of Rock Polish in SV (hallelujah, I definitely think it could get banned if it still had Rock Polish), though I still like mixed Swords Dance with Eruption which is an absolutely horrific wallbreaker. Bulky SD sets are also exceptional with Precipice Blades, Rock Tomb / Stone Edge / Dragon Claw, Swords Dance, Stealth Rock / Spikes / Heat Crash / whatever. Obscenely good mon that defines the meta and really doesn't have any reason to be dropped.

My personal pick for the #2 mon after Primal Groudon. Now that Miraidon is banned, Mega Rayquaza has stepped up as the premier offensive threat. While I've seen a lot of people express the sentiment that it'll be too much, I don't really agree as it doesn't lack competent counterplay with how the meta is shaping up. The biggest complaint I have against Ray is that it's a
really potent Tera abuser, but I'll talk about Tera later. Choice Band, Dragon Dance, and mixed sets all seem very solid though all with their own issues. As for answers, the loss of V-create means that Dragon Dance sets are unable to OHKO physically defensive Necrozma-DM and it's pretty walled by Skarmory unless it's a mixed set, and physically defensive Arceus formes are solid answers as always. Ray's main issue is that it struggles to find opportunities against the types of structures that people are using now, with Will-O-Wisp Primal Groudon, defensive Arceus formes, Extreme Killer Arceus, and Zacian-C all being prevalent, not to mention Tera Steel Giratina-O being incredibly consistent into Ray overall.

Another very splashable mon due to being one of the only relevant hazard removal options. It checks Primal Groudon quite well and is bulky enough to take hits from most pokemon, and its synergy with Tera Steel and Levitate is incredibly potent, especially when it comes to answering Ray. I've personally been running Calm Mind, STABs, and Defog the most as it's the best at consistently removing hazards against Primal Groudon and can be quite the threat in its own right. Stuff like status move + Hex could also be good but it's much worse at handle Primal Groudon which is why I've not really used it too much, could be good with other Don answers like Groundceus or defensive Primal Kyogre.

Speaking of, this mon is crazy. Much like Primal Groudon, gaining Leftovers is absolutely insane for its longevity, and it really doesn't have any great checks besides Primal Groudon which gets worn down by Ice Beams scarily fast, not to mention having to flee if Kyogre is the one coming in on it. I think offensive sets aren't the play as defensive sets are genuinely heinous to deal with due to how fast they snowball with CM boosts. I prefer Rest to the Protect sets that Chessking likes as it's just so good being able to fully heal and absorb status, especially with how many free turns Ogre can get with its bulk.

One of the main things I've really liked so far. I've tried both offensive and support sets and they both felt very solid. The weakness to Earthquake means that you can't tank +1 Mega Rayquaza so you can run max Speed without any qualms. Flying-type resistances are pretty much necessary to not get rolled over by CB M-Ray, and Steelceus is a very splashable fit for the roll. Being good into Diancie is nice for ladder too and Steel-type resistances are just good to have.
:sv/-arceus-dark:
The other Arceus forme that fufills a unique roll in the meta that I've loved using. STAB Foul Play is
sooo good at keeping physical attackers in check, even resisted ones with how strong many of the threats in the meta are (one of my favorite moments on ladder was clicking Foul Play into a Mega Blaziken as it clicked SD and outright OHKOing.) It can either run max defense to take +1 Dragon Ascent, or a faster spread—I ran enough Speed to outrun Adamant M-Ray for example—and be effective either way. The move freedom that is granted by Foul Play working well enough against physical attackers to not run Will-O-Wisp means that you can run a variety of different moves in your open slots. Will-O-Wisp, Thunder Wave, Taunt, Dragon Tail / Roar, Stealth Rock, Sludge Bomb / Poison Jab, Healing Wish, Perish Song; all of these are options that are worth considering and can open up new possibilities when teambuilding. I personally ran Healing Wish and absolutely loved it; it gave me so much more flexibility when it came to answering threats because I could heal up my Primal Groudon or Giratina-O, or give a weakened / crippled M-Ray another chance to wallbreak, revenge kill something, or even win outright with Dragon Dance. Also, Taunt Calm Mind sets are incredibly annoying especially with Tera.

The premier Arceus forme when it comes to purely defensive sets. Even though it is hurt by the loss of Toxic leaving it often struggling to make progress against Primal Groudon and especially Ho-Oh, the defensive qualities really are so good that it doesn't matter. Being able to OHKO Mega Rayquaza with minor chip (or a Dragon Ascent drop) is something that only Fairyceus, Dragonceus, and Iceceus can do, and Fairyceus is notably better than both of the others (though Dragonceus is still respectable imo.) An immunity to Dragon-type moves is also very useful, in addition to the Dark- and Fighting-type resistances that allow it to easily handle Gira-O, non-CM Darkceus, and Tyranitar, while obviously being the best answer to Koraidon available. Offensive CM sets can also be incredibly scary into teams without Ho-Oh, Eternatus, Chansey, or Shuca NDM.

Extreme Killer looks to be
very good in my opinion, and I'm surprised I haven't seen too much of it. Being able to revenge kill M-Ray decently is very nice, though if you're a faster set you're at risk of getting killed by Tera Normal Extreme Speed after a bit of chip damage. The lack of Marshadow in this metagame is very good when it comes to using E-Killer as a win condition as there's no faster Ghost-type that can blow it up without a Chople Berry.

That's a lot of Arceus formes in a row, so let's talk about something else for a change. Zacian-C is very solid and quite splashable, especially on offensive teams. I disagree with Chessking putting it in his top 5 but it's undeniably good. Primal Groudon can be very annoying for Zac-C to break without expending its Tera (and even then it's pretty difficult.) I've seen Tera Ground and it's pretty scary but I think Tera Fighting is better for its ability to hit P-Don hard without Teraing, in addition to OHKOing Extreme Killer and hitting Tera Steel Giratina-O. My earlier statement about needing a Dragon Ascent resist holds true even for offense, and Zacian-C is often the best mon for the job, with its speed tier also allowing it to keep a few other notable threats like MMY and Koraidon in line for its team, with its typing being especially helpful for pivoting into both of those attackers.

Ho-Oh is a massive pain. It's bulk, typing, and Regenerator allow it to stuff mons unlike anything else in the meta, and those traits also allow it to soft-check an absolutely massive number of pokemon. Both Primals and Mega Rayquaza can make life difficult for Ho-Oh but it's more than capable of getting around them to harass a number of top options like Fairyceus, Steelceus, Groundceus, Giratina-O, Zacian-C, Koraidon, and Darkceus.

I've been pleasantly surprised by Diancie's performance as a lead, with a lot of different options that all work in tandem to enable the dangerous offensive teams it's found on. My personal favorite set is Moonblast, Earth Power, Rock Tomb, and either Spikes or Stealth Rock depending on the rest of the team. Outside of the lead slot though... Diancie has been disappointing to say the least. It's really just not strong enough or fast enough or bulky enough.

Easily the best supportceus when it comes to answer Primal Groudon, and boy is that a big deal. The resistance to Stealth Rock is also quite notable as you don't have to worry about dying to a +1 Dragon Ascent high roll after rocks. Unlike Fairyceus, Groundceus can't OHKO -1 Ray with Ice Beam without it taking Stealth Rock damage, which is quite annoying, but the incredible Primal Groudon matchup, as well as being good into Zac-C definitely means that it's worth running on some builds. Offensive Calm Mind sets are also absolutely ridiculous to deal with if the opponent doesn't have Ho-Oh, Primal Kyogre, or Tera Steel Giratina-O. I haven't tried Dragon Dance yet but it seems like it struggles greatly against a lot of common options like Giratina-O or Will-O-Wisp Arceus formes. Overall one of my personal
favorite mons to use, even if it's not outright the best.

Maybe the best purely offensive Calm Mind Arceus forme. The perfect coverage of Judgment and Aura Sphere / Focus Blast makes it very difficult to answer consistently, especially with its hard-hitting Judgments being quite strong even into Ho-Oh's high bulk. Similar to Extreme Killer, it benefits greatly from the lack of Marshadow in the metagame to comfortably revenge kill it, and both the immunity to Extreme Speed and Arceus' high natural bulk make it difficult to revenge killer.

One of the coolest mons IMO. Just like Darkceus, STAB Foul Play is great at chunking pretty much all physical attackers, and between its frankly massive bulk, good typing, and potent movepool it has a lot of use, especially as one of the best answers to Mega Rayquaza. Knock Off and Stealth Rock makes it a solid progress-maker, and it can spread Paralysis with Thunder Wave. It's so bulky that it doesn't even need to have Shuca in order to take a boosted Earthquake from M-Ray so it can experiment with other options like Leftovers or Chesto Berry. Overall just a really cool mon.
In case you haven't figured it out by now, Arceus formes are
really fucking good; pretty much everyone should be using them a lot more than they are at the moment. Overall I like the current state of the meta, as I think both bulky offense and balance teams are very good