Bringing up one of my previous posts; my opinions about these Pokemon still stand and Grass Arceus in particular seems to be discussed pretty hotly.
I've developed some new opinions since I last posted here. This is on top of what has already been discussed.
Grass Arceus for B
Not a B- mon. Checks Kyogre, possesses useful resists to Ground and Electric, and can viably run one of several coverage moves to handle one or another of its checks, depending on team needs. This is more B+ in my head but at any rate it needs to get out of B-, where it's ranked lower than the likes of Heatran.
Steel Arceus for C+
The Fire, Fighting, and Ground-type weaknesses speak for themselves. The SD set is way outclassed by other Arceus formes, while you would rather use Heatran in any other scenario.
Water Arceus deserves to be one sub rank above Electric and Poison Arceus, which in turn deserve to be 1 sub-rank above Fairy Arceus.
Water Arceus is a phenomenal glue mon, checking to varying degrees Ground-types, Ho-Oh, and Kyogre. It succumbs to the latter 2 in the long run, but that is besides the point; it does brilliantly at holding them off in the interim, giving you a lot more room to pivot and turn momentum in your favor. I view the support set sort of as I would support Dialga; both don't really kill things but provide a brand of team support no other Pokemon can even come close to replicating. There is also the CM Refresh set, which is very threatening in its own right and gives it a another distinct niche. I'm hence nominating it for A.
Eleceus is solidly A-; it is a very solid CM forme but this is balanced by its one-dimensionality. Poison Arceus also deserves A- and Fairy Arceus B+ for reasons Fireburn and Orch delineated (respectively).
So yeah, I'm strongly advocating
Grassy for B+ due to reasons delineated above. I actually feel it's marginally more useful than Fairyceus. Notably, its Ground- and Electric-type resistances are often overlooked. It's also a brilliant Defogger, effectively runs WoW/Toxic/Twave depending on team needs, and along with Waterceus, is pretty much the only Pokemon that can switch decently well into both Groudon and Kyogre. The Fire-weakness and susceptibility to Yveltal and Mega Gengar relative to other Arceus formes prevent it from rising any higher than that, though.
Steel Arceus is really bad; I'm thinking it's
C+ at best. CM Steelceus is a legitimate late-game threat, but that is literally the only set worth using, and weaknesses to 3 potent offensive types in Fire, Fighting, and Ground means that that suffers from huge opportunity cost anyway.
Water Arceus really deserves
A- ---> A imo for reasons I articulated in the quoted post. It's neutral to almost
everything, which is more than most support mons can claim, essentially possessing only one troubling weakness to Electric-types and is the go-to Defogger/pivot for a huge number of balanced builds and many stallish ones as well. As I said, I view Waterceus as an analogue-of-sorts of Dialga in that both have great, irreplaceable niches.
Other proposed changes:
Sylveon: B+ --> B
Solid Yveltal + Palkia check that additionally provides Wish + cleric support. At the same time, it is acutely one-dimensional in the sets it can run, sorely misses the utility of a status move, and faces competition from other Fairies. None of these are Sylveon's faults per se, but it does mean that B+ is a shade too high. Since Fairy Arceus, which offers solid support and is on the whole a far more versatile mon, is getting B+, Sylveon deserves a sub-rank lower.
Reshiram: C --> B-
The Toxic + Roost @ Flame Plate set alone justifies this. It can beat out all common support Arceus barring Rock, can switch in (double or otherwise) on Steel-types barring Heatran, and Draco Meteor + Blue Flare make it far from easy to switch into. The threat of Blue Flare alone forces very predictable switches that can be exploited by strategic doubling. The other sets are pretty shitty (Flame Charge + Roost/Stone Edge/Toxic + STABs is a decent cleaner) but overall C-rank is just unacceptable.
EDIT: Webs setters: C+ --> B-
The fact that Sticky Web is very much relevant in this meta + the fact that Shuckle/Smeargle are indispensable on these teams justifies them getting a B- rank imo. Shuckle is the more reliable setter, but Smeargle offers more utility by way of Spore/Nuzzle/Rapid Spin, so it evens out.