Haruno
Skadi :)
Opportunity cost is one shitty argument. If you're running ekiller then you lose out on running cm ghostceus or sd ghostceus or support ghostceus or hail iceceus or sd dragceus, etc. So regardless of the form it still has massive opportunity costs. Not to mention we're rating mons as a whole not individual sets. SD/filler/filler/filler normal ceus is one of the sets normalceus can run it isn't normalceus itself.I agree with what you said on the ranking structure, as S-Rank should be the pokemons that define the metagame. (Basically, Kyogre and Groudon.)
However, I won't debate on the ranking of Arceus-Normal. For me, he's A, because the defensive set has so much opportunity cost. Wallceus does definitely not worth A-Rank. As a wall you have much better choices in the metagame, such as Skarmory, Groudon, Hippowdon, Landorus-T...and you sacrifice another form of Arceus to it.
Now, Arceus-Ghost is definitely S-Rank, not because he shapes the meta, but it's a ridiculous pokemon on his own. The combination of high bulk, high speed, ghost-typing and decent offense cannot be found into any other pokemon. He can set up CM on pretty much 2/3 of the pokemon in Uber, and forces to use counters (they are fewer than extremekiller's counters!) such as Darkrai, SpDef Kyogre, Tank Ho-Oh, Tyranitar (only once). That's right, only four counters. (I won't count Arceus-Dark) Kyogre may be more useful when running a choiced set, depending on the team, Darkrai cannot take a Focus Blast, and Ho-Oh is hard to fit in a lot of teams, Tyranitar brings sand which can be disadvantaging for your own team. His has a lot of utility: SR+Spinblock(who, in Uber, can do this?), Magic Coat, ability to beat spikers (all of them).
His defensive set has huge advantages over the standard normal wallceus. He can spinblock, he has a better typing, and runs 252+ in speed, which means he can burn a lot of pokemons before taking an attack, unlike Wallceus. Not to mention his immunity to Extremespeed (Hi SD Ray and Arceus!).
Overall Arceus-Ghost is very effective in the metagame, and shouldn't be anywhere else than S.
As for your ghostceus argument, he lacks an item bar spooky plate. Not to mention that 252+ speed statement makes no sense whatsoever. Smogon's analysis sets aren't the only sets out there. What stops normalceus from running lefties/scarf or whatnot to help cripple attackers or prolong its longevity? Nothing unlike ghostceus. So if you're really going to make an argument for ghostceus please don't use some one sided crap. Bulky arceus isn't that sturdy just fyi.