This is how I suggest
A rank to look like:
Latios: Checks both Primals, hits really hard, has great coverage, and has Defog. My favorite set is 4 attacks and it's really great.
Deoxys-Speed: This Pokemon's ability to put Stealth Rock on the other side of the field is pretty scary. The worst case scenario is a Magic Bounce Pokemon on the opponent's team which can be played around using Skill Swap. However, that's not the main reason why it should top the A rank. When paired with Ubers threats specially Darkrai it really restricts team building and makes Darkrai and many other offensive threats very difficult to handle with Stealth Rock up.
Sableye-Mega: The Pokemon that makes stall viable. It walls lots of Pokemon that would otherwise annoy stall with their hazards and status spam like Skarmory and Ferrothorn. It checks top tier threats like Extreme Killer Arceus, Mewtwo, and Deoxys-A. It also drag the opponent into 50-50s specially when he wants to use Stealth Rock and Toxic and when he wants to use Dark Void with Darkrai.
Arceus-Ghost: Checks Extreme Killer Arceus and hits really hard at +2. It's also immune to Extreme Speed and has Extreme Speed which makes it difficult to revenge kill. Calm Mind and Defog bulky sets are not bad either. However, It's weak to Darkrai and Yveltal who totally blocks it and it also comes at the opportunity cost of not using Extreme Killer Arceus on offense.
Arceus-Ground: Great offensive Pokemon. It has a huge sweeping ability with Ground STAB and also provides defensive synergy by checking Mega Diancie and Primal Groudon. However, it can be revenge killed pretty easily by faster Pokemon like Darkrai, Shaymin, and Mewtwo.
Klefki: Good at team compression since it checks a huge portion of the metagame. It can also be used as an emergency check against some Pokemon like Mega Salamence. However, it's a free switch in to juggernauts like Primal Groudon and Ho-Oh while Block Rest Xerneas is more popular now so
It should drop to A-.
Mon amour je suis très desolé, mais je ne suis pas d'accord avec toi sur ça. xP Je t'adore, bien sûr, mais maintenant, laisse moi expliquer pourquoi je vois les choses différemment.
First of all, I think A rank should be ordered in the following: Mega-Sableye, Deoxys-Speed, Latios, Arceus-Ghost, then Klefki, whilst dropping Arceus-Ground to A-; here's why I think this way.
Before anything, I pitted Deoxys-Speed and Mega-Sableye against each other for the top two spots, and decided to give the upper hand to Mega-Sableye. This is because the two act as antithetical counterweights to each other in the ORAS metagame in guaranteeing either a successful execution of or prevention of Stealth Rocks being set upon the field. While Deoxys-Speed will always get Stealth Rock up against Mega Sableye, the Magic Bouncers more than makes up for this with its myriad of utility functions it brings to the table such as checking the most dangerous sweeper in the game, Ekiller, and rendering a good chunk of the low-offensive-presence façade of the metagame useless, like Ferrothorn, Klefki, Skarmory, and Tyranitar, and even those with higher offensive presences like Giratina-O, Mega-Kangaskhan, Mega-Mewtwo-X, defensive Yveltal, and Landorus-T. It can even come through clutch against Pokémon like Deoxys-Attack and Mewtwo, too. At the same time, on the other hand, we have Deoxys-Speed. While just the fact that it guarantees Stealth Rock alone should have given it its top spot in A (hell, these two Pokémon are so good I don't mind putting them both in A+), I believe that it is perhaps even more important to understand that without the existence of this Pokémon, Darkrai would not be even close to its offensive capacity it currently possesses. If Deoxys-Speed isn't at the apex right under Sableye-Mega, you might as well drop Darkrai's viability too since it's a really one-dimensional Pokémon with great offensive synergy with Deoxys-Speed. So yeah, the fact that these two are absolute obligatory Pokémon despite them both having opportunity cost to an extent (the former being a Mega and the latter being a Deoxys-form) really drives home just how staple these Pokémon are in shaping how principal play-styles are strategized in this metagame. tldr I am really behind an ordering of, one, Mega-Sableye, and two, Deoxys-Speed in A rank, so much so that bumping them up to A+ wouldn't get any complains from me. I think whether this is a correct train of thought should be debated first before going to the other A rank Pokémon.
This brings us to the rest of A rank, and I found this to be more difficult to sort compared to how intuitive placing the aforementioned two in the top slots was. First, let us reexamine the very definition of an A rank Pokémon's viability: "This rank is reserved for Pokemon that may have great offensive or defensive capabilities. They tend to be the first to be considered for certain roles, but may need some support to function well." While none of the four are clearly outstanding in defensive capabilities as bigger dogs like Primal Groudon, Arceus-Water, and Lugia are, I hope it is quite obvious to you all that Latios takes the cake in offensive capabilities. Lord Outrage blatantly placed it at the top of this rank and isn't not like I don't understand where his sentiment roots from; not only does this thing bear with it insane offensive presence, it can also technically perform supportive roles like Defog, spread Thunder Wave, increase staying-power with Roost, and suicide with a Memento; with a flexible movepool both offensively and defensively (and, admittedly, bad 4MSS), Latios is a notch above Arceus-Ghost and Ground in terms of viability thanks to the immediate damage output in a nuclear Draco Meteor alone, along with it a variety of unique supportive roles it can adapt to according to one's team's needs. Additionally, and perhaps equally as important, Latios is an extremely solid check to Primal Kyogre. This is important because just by adding Latios on your team, one gains the freedom in allowing for Primal Groudon to play a role outside of it being a Primal Kyogre check, leading it to more comfortably take on the plethora of threats it must face each battle such as Xerneas and Ho-Oh, intrinsically expanding the team structure's potential for innovation.
Next, I chose Arceus-Ghost. This is because it's carved out a niche as an alternative to the best Arceus form, Ekiller, as a Swords Dance sweeper, whilst also being a spin-blocker and a Ekiller check. As a result, this Pokémon is very easy to fit on offensive teams using Toxic Spikes, as Giratina-O is unfavorable on such builds, and slower offensive teams that may benefit from having a spin-blocker as it is not able to put pressure on the opponent to not spin as much as the standard hyperoffensive team does. Though, it also does not go without saying that this Pokémon is plagued with some serious flaws, to a point where I see the divide between the true kings of Ubers and the more shuffle-able Pokémon starting around here. First, the Swords Dance set pales in comparison to Ekiller neglecting the small utilities it provides like spin-blocking, checking opposing Ekillers, and serving as the team's imposter-counter, as Shadow Force is unreliable and does not do much until Arceus-Ghost has found a window to set up at least once, and its Extreme Speed is pathetically weak due to a lack of STAB. Furthermore, Arceus-Ghost is usually completely cockblocked by Yveltal, and even if you run Stone Edge, which you shouldn't since Brick Break is a lot better, Sucker Punch hurts big time. Finally, I've never been a fan of the Will-O-Wisp support set; I never see a reason to use it on balanced sun teams over Arceus-Water, it falls flat on its face against Recover and Refresh variants of Ekiller and Facade and Refresh variants of Mega-Salamence, and 85% accuracy isn't the best thing in the world to rely on as a primary countermeasure to such menacing threats. Credits to the SubCM Judgment variant, though, it works surprisingly well albeit, again, being stopped cold by stuff like Darkrai and Yveltal.
Finally, I chose Klefki. On top of everything Lord Outrage said, Klefki is pretty one-dimensional in its function... ...but, I think Outrage was a bit harsh on this thing. While I wasn't convinced by people saying Giratina-O was a good compressor, I do believe that Klefki is a great compressor. Mainly, its ability to cold turkey Darkrai, Yveltal, and standard Xerneas and Mega-Salamence is just amazing. Finding a place on a team is pretty easy thanks to the plethora of Pokémon it checks with its awesome typing and Prankster Thunder Wave, while Heal Block variants allow it to keep Xerneas from abusing it as set-up bait. Counters like Primal Groudon and Ho-Oh despite getting hit by Toxic, too, and I generally view this Pokémon as a subpar but justifiable way of keeping a competent balance team together; the definition of a good "glue" Pokémon.
And lastly, we have Arceus-Ground. I'm going to be honest, I think this thing is grossly overrated. On paper it looks great, being the only Arceus form to jab at Primal Groudon supper effectively with its STAB, but empirically, I have been extremely disappointed with how well this Pokémon fits on teams. Before anything, I want to establish that this thing is not your sole answer to Primal Groudon; the Swords Dance set running Jolly fails to KO even the frail PolishDon, which can in turn 2HKO you, while the support variant is a good counter to PolishDon, which unfortunately despises burns. While the difference between the support variant and Arceus-Water might not seem like much, the extra damage from taking Lava Plume neutrally really stacks up. :/ Its moves are awkard too, for both variants. The Swords Dance variant wants to be a Primal Groudon check but can't come in on support variants fearing the burn, rendering it useless as a Primal Groudon check (in that case, why not just use Ekiller, right?), while the support variant has a hard time justifying its position over Arceus-Water very often. As a result, I find it to be one of the lesser effective Defoggers in the tier, and in conclusion, I don't think it's really fit to find itself in A rank to begin with at all...
So yeah tldr Mega-Sableye because it's obligatory on stall, prevents hazards, and checks more things than any other Pokémon in the metagame individually, Deoxys-Speed because it's obligatory on hyperoffense, guarantees Stealth Rocks on the opponent's field, and exponentially expands the threat that other offensive Pokémon in the tier pose as a result, Latios because it checks Primal Kyogre well and thus finds good synergy with an omnipresent partner in Primal Groudon, its immediate offensive presence is a notch above the others, and its unique supportive roles are exploitable and unpredictable, Arceus-Ghost because its Swords Dance set finds an established niche on certain teams and holds potential to sweep, albeit it is marred by banes like Darkrai and Yveltal, Klefki because it's an excellent compressor and can halt the biggest threats in the tier from sweeping you, although its typing is a double-edged sword coupled with its underwhelming stats making it easily counter-able, and I nominate Arceus-Groud for not really being great at what it tries to accomplish, both as a Swords Dancer and a support set, the former often being outclassed by Ekiller or Ghostkiller and the latter often being outclassed by Arceus-Water and it not being too effective in the first place.
Sneak peak at sorting A- rank I think Diancie should definitely be the at the top in this sub-rank.