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.