Alright, here are my opinions on Dialga and Ghostceus:
Dialga should stay A Rank. Dialga is excellent at setting up SR. In fact, it's one of the best users of the move, if not THE best. But I don't think that in itself is enough to warrant a promotion. Although Dialga's defensive qualities are awesome, they are often somewhat overstated. While Dialga softchecks a lot of Pokemon, it's not really an outright counter to anything either aside from Eleceus. As a Zekrom check it is somewhat limited since Zekrom can just Volt Switch to a Ground-type to punish the opponent for sending it in. It can also only really come in on Palkia once since Dialga is neutral to Dragon and 100/100 special bulk only carries it so far. Relying on Dialga as a Kyogre check is also risky - what if that Kyogre you thought was Scarfed is actually Specs or Specially Defensive? Although the fact Dialga can even check these Pokemon at all is amazing, it's important to remember that it is only a softcheck - and it will be worn down quickly. Dialga is also unable to check Xerneas or Yveltal that well which is annoying since they are extremely prominent threats in the metagame, and a lot of things Dialga was good at checking in the past (Genesect and weather sweepers) see far less usage nowadays.
Dialga is also rather easy to punish for most offensive/balanced teams given the infestation of Ground-types and strong Mega Fighting-types like Blaziken, Mewtwo-X, and Lucario in the metagame. While Groudon usage has fallen a bit as someone pointed out earlier, Landorus-T has risen to take its place, and Gliscor/Groundceus/Excadrill are ofc still around. Given the prevalence of Zekrom, having a Ground-type on the team is more important than ever, which constrains Dialga's ability to do work.
All things considered, Dialga is an excellent Pokemon and totally deserves A rank, but its not quite A+ level imo.
I am fine with moving Ghostceus to A+ Rank. A lot of people forget that Ghostceus received two important buffs in Generation 6. The first is the fact that Steel-types lost their resistance to Ghost, which makes Ghostceus a lot more potent offensively. The second is that Ghostceus is immune to Shadow Tag, making it an excellent and often underrated support Arceus forme that can check Extremekiller (check to whom are in short supply), Mewtwo, and somewhat Blaziken (Mega Flare Blitz still 2HKOes without rain support), all of whom are major metagame threats, without fear of Mega Gengar. It can also swap in against threats such as Dialga and Palkia somewhat comfortably since they often cannot hit Ghostceus for super-effective damage. Ghostceus is one of the most versatile and threatening Arceus formes in the game, able to pull off CM, SD, support and even hybrid sweeper/support sets to great effect.
So why shouldn't it be S-rank? Simply put, there are also a lot of metagame changes that have negatively affected Arceus-Ghost, enough to where it is no longer an S-rank metagame defining force. Simply put, a lot of Ghostceus checks are better than they used to be. Kyogre thrives in a metagame without Lati@s (which is also a very important mon Ghostceus could check!), and most Ghostceus sets save SD struggle hard against it. Defog, although it has helped Ghostceus since it learns it, has also harmed it since it makes the spinblocking niche much less important and makes Ho-Oh, an old Ghostceus check, much more of a threat. Compound this with the weakness to Darkrai, a very powerful new offensive check in Yveltal (which owns every single Ghostceus variant) and the extremely Toxic/Wisp heavy metagame in this generation and Ghostceus has a lot less wriggle room than it did before. It is important to remember that Dark-types are also a lot more common than they used to be since they were buffed as well.
Yveltal in particular is a huge menace. Normally, type-neutral or CM Arceus formes like Electric, Water, Dark, or Fairy can help considerably against checking this thing. Ghostceus is countered by it.
"But can't Ghostceus just use Thunder to break Ho-Oh, Kyogre, and Yveltal?" Not without giving up Fighting coverage or WoW, thus costing it its ability to check EKiller! This also shows that, although Steels were nerfed, Ghostceus can't afford to give up the Fighting attack or WoW that helped it combat those guys since otherwise it fails in checking EKiller (once of the main reasons to use Ghostceus in the first place).
"But don't other S rank mons like Kyogre and Xerneas have hard checks?" True - Kyogre can't do much to Gastrodon and Xerneas struggles against Aegislash. On the other hand, Gastrodon and Aegislash pretty much do nothing outside of checking those Pokemon and Toxicing stuff. Yveltal is a common A+ rank offensive threat that is extraordinarily menacing on its own and used for many other reasons besides checking Ghostceus. It is also rather hard to get rid of because of 2 possible recovery methods, a lack of solid lures for it (other than like Ice Beam Mewtwo), and the threat of it smashing your face in.
So in short, while Ghostceus is an excellent and often underrated Pokemon, it suffers from a lot of its checks being better, a new check-going-on-counter existing in Yveltal, and more competition from other Arceus formes (EKiller is better this gen, other Arc-types that can check stuff Ghostceus struggles against are in higher demand). It is still a huge threat every team should prepare, but it is no longer a metagame defining force in the way Kyogre, Xerneas, and the other S-Rank Pokemon are.
I hope this is clear, its a bit tl;dr lol.
