These Pokemon were by far the most dangerous restricted choices, being very difficult to check with normal Pokemon.
Mega Rayquaza
Why was it so feared?
This thing was so strong that even Ubers banned it! It was no joke in last year's special either. This thing is damn near impossible to wall, with most offensive teams having very little of anything resembling a competent switch-in to this outside of Focus Sash cheese. Mega Rayquaza's high power, speed, and ability to hold an item put it WAY above any other Mega Evolution choice, with others only really being used if a different restricted slot is chosen!
A lot of people assume you'll just Dragon Dance with Life Orb, but note that Mega Rayquaza is capable of more than that! While Focus Sash may seem super silly for a Pokemon with Mega Rayquaza's bulk, it can act as a nice anti cheese against opposing Focus Sash Pokemon. There's the occasional Swords Dance set, which I don't feel is quite as good as Dragon Dance because Choice Scarf Tapu Lele was everywhere, but it was even harder to wall for defensive Pokemon! Then there's mixed Mega Rayquaza, who puts floating Steels to utter shame, and plus, Draco Meteor hurts a hell of a lot! Choiced items aren't shabby either, with Choice Band having an incredible amount of immediate power, and access to V-create which is actually just as strong as Dragon Ascent! There's even Choice Scarf, which I didn't see much of, but was really cool for blasting Tapu Leles that may attempt to revenge Mega Rayquaza.
What's changed this year?
I think the the big change for it are going to be Marshadow. I expect a lot of Marshadow to carry Focus Sash, which will really suck if Mega Rayquaza sets up, since even +1 Spectral Thief from Focus Sash Marshadow actually 2HKOs Mega Rayquaza, and you can't just ExtremeSpeed it either! Ultra Necrozma might be a bit weird for it to. I don't expect many to actually run Dragon-type moves, but note that if it does end up using even Dragon Pulse, it can outspeed and OHKO unboosted Mega Rayquaza. While I'm not sure this is going to be common, I imagine Focus Sash Naganadel might also be a big issue for Mega Rayquaza due to its superior speed. Otherwise, I think Mega Rayquaza going to be mostly as good as it was last year!
Arceus
Why was it so feared?
Just speaking about Normal Arceus alone, this thing is a nightmare to face for hyper offensive teams, especially when paired with Tapu Koko to get rid of Psychic Terrain. Boosted ExtermeSpeed just cleaves through fast offensive stuff, and thanks to Normalum Z, Aceus can even severely damage the tankier stuff in this format too! I think Normal Arceus is easier to wall than the other two S rank Pokemon, but it deserves this spot just because its such a menace against offensive teams, and is a major reason Tapu Lele was so popular (the other being Mega Rayquaza lol).
boe's stats if I recall didn't record Aceus totally correctly, but from my play, other Arceus formes were uneruitilized and under explored, which is a shame because they can be nasty surprises due to team preview not revealing them! The three that stood out to me the most where Fairy, Steel, and Ghost. Fairy is of course, a brilliant typing, and was handy on a Calm Mind set for blasting Mega Rayquaza, though being weak to common Steel-types was an issue. Steel is an excellent defensive typing that Arceus could use offensively, and it did extremely well against Mimikyu and Tapu Lele (just watch out for Scarf or Sash Focus Blasts), while Ghost was unique in being the other type that could effectively use a Z-move, thanks to +2 Z-Shadow Force.
What's changed this year?
I think Marshadow has the potential to be a real bummer for Normal Arceus, since Marshadow is faster and slams it with Close Combat. I think it will also totally obliterate Ghost Arceus usage too thanks to Spectral Thief shenanigans >_>. Fairy Arceus is also going to be rather awkward against Dusk Mane Necrozma, while Steel is also going to dislike Marshadow running about.
That being said, I think there could be other interesting Arceus types to try out, such as Ground to deal with the Steel-type spam. I think Arceus is the one that's the most open to experimentation for sure, so feel free to use an unusual forme if it fits your team!
Kyogre / Primal Kyogre
Why was it so feared?
Primal Groudon was actually a lot tamer in this format compared to say, Ubers, and Kyogre benefited from that pretty massively! Because Primal Groudon wasn't a super common restricted slot choice, Choice Scarf was actually the most common kind of Kyogre! With just a rain boost, Water Spout is a freaking nightmare to switch into with unrestricted Pokemon, with Ferrothorn being the only particularly good one. Restricted wise, you got Mega Rayquaza and Mega Groudon as good choices, though even the former won't like a predicted Ice Beam!
Choice Scarf isn't all Kyogre does of course, since Primal Kyogre is still a huge threat on its own. Its extremely specially bulky so it can prey on stuff like Choice Scarf Tapu Lele locked into weak moves, and can Calm Mind on threats such as Aegislash or Celesteela. While not super common, I'd also watch out for Sheer Cold Kyogre, who can REALLY ruin your day if it hits Ferrothorn or Primal Groudon!
What's changed this year?
Honestly, I think the only change Kyogre has to worry about this year is Ultra Necrozma, since before it Ultra Bursts, Kyogre can be stuck in an awkward prediction war of "should I use a water move to predict it to stay in its forme, or should I Ice Beam and predict the Ultra Burst?". Otherwise, I imagine things will be business as usual for Kyogre.