Alright, I will go a in depth on each of these Pokemon.
Rotom-W is, imo, a clear A rank by now. It was a definite S rank back before Pokebank was released due to the popularity of Talonflame, T-Tar, Mawile, etc., but I doubt its performance is S rank-worthy, now that there is a wider pool of Pokemon to choose from. The metagame has, by now, developed to revolve around really powerful Pokemon such as Mega Charizard Y and Mega Kangaskhan, and Rotom-W just does not cut it as a bulky mon with its pitiful base 50 HP stat. Sure, it checks Heatran, but even they have started to run Speed enough to outpace Rotom-W and "stall" it out with SubProtect (By "stall", I obviously mean kill it with your partner, lol actual stall sux in dubs).
With its godsend of a typing, it remains a powerful glue to almost any team out there, facing very little competition from other Pokemon of its typing(s), but its overall stats don't really complement this amazing typing. Furthermore, the stats don't really complement each other that well either; its defensive stats are good, but its HP is terrible, its Special Attack stat is good, but its Speed is middling. This obviously won't stop Rotom-W from fulfilling its unique role, but it does limit it to a potential lower than that of amazing mons such as Mega Kangaskhan or Cresselia who perform their roles almost optimally and always consistently. I think we need to face cold fact: its stats honestly just don't cut it for S-rank. Rotom-W from S to A rank
Amoonguss is definitely no A rank. Fire-types are all over the place in this metagame. Unfortunately, the Spore and Rage Powder nerf also no longer allows it to counter Grass-types, and Grass-types can now even take advantage of Amoonguss' presence. I'd say you'll need gigantic problems with Washtom if you are going to be having this thing on your non-TR team. Honestly, I see no justification for this Pokemon to be in a tier amongst titans like Terrakion, Landorus-T, and Genesect. Amoonguss from A to B rank
Gengar is definitely the toughest choice from these four nominations. I know I nominated it for A myself, but now I'm not so sure anymore. It does have a fantastic STAB attack and a cool Mega Forme with an outstanding ability, but it is unfortunately extremely frail. Shadow Ball is also not a very high BP move, so despite having amazing SpA and great coverage, sometimes it's just not powerful enough to nab those OHKOs.
It's best merit is its ability to counter the living shit out of Trick Room. With the combination of STAB super effective hits for all of Ghost-types, Psychic-types, and Aromatisse, both of its formes can prevent TR from being set very efficiently. This is obviously a big help to many fast, offensive Pokemon, including Gengar itself. I am going to vote blank on this nomination for now, as I believe I need a little bit more experience with using it before I can convince myself it should be moved. Gengar staying in B rank
edit: shit I got ninja'd on this one :( ^
Volcarona is easily A rank in my honest opinion. It has two solid roles, and both are performed very well by it: Offensive Rage Powder supporter, and late-game Quiver Dance sweeper. With new mechanics and Pokemon that greatly help Volcarona in doing its job of sweeping (Wide Guard / Quick Guard buff + arrival of powerful Fake Out user Mega Kangas), and even though not as good as it could be, still can use Rage Powder to good effect, completely cockblocking Fighting-, Grass-, and Bug-type attacks, as well as being generally specially bulky.
The only reason I could see justification for it staying in B rank is because some may say it is overshaded, or even outclassed, by Heatran and MegaZard Y in many ways, due to having similar roles and resistances, but those two Pokemon can't late-game sweep nearly as well as a Volcarona with the right setup That said, I think Volcarona easily has two roles unique and solid enough to warrant an A rank Pokemon. Volcarona from B to A rank
Holy shit I was not expecting to write so much about these Pokemon, but there you have it =]