I think A+ should go Mega Gengar, Darkrai, Arceus-Water, then Lugia. Sweep I agree with the points you made and I understand where you're coming from but I feel as though you've failed to mention a few things. First, regarding Arceus-Water and Lugua; I believe that while Lugia by virtue of its ability, typing, and stats, is able to take on threats with much more comfort given ideal circumstances. Yet, one must also bear in mind that Pokémon is a game about what happened and not what could have happened. While it can take on offensive Primal Groudon better, Lava Plume burns take away its Multiscale, and while it can take on standard Mega Salamence better, the Refresh variants can beat it while Arceus-Water's Ice Beam at times outdamages Lugia's to a point where the dragon falls (this depends on your opponent's spread, though). It is also worth noting that while both can run purely glue-esque sets (Ice Beam and Toxic versus Judgment and Toxic), the latter can also provide crucial defensive support in the form of Defog. Furthermore, Arceus-Water can find a fit on both balance and stall teams, changing up its moveset to accomodate for certain threats or to become the team's Defogger, making it versatile and therefore more viable, Lugia can really only run one set. Basically, Lugia finds a home on stall teams better than Arceus-Water does, but in reality, Lugia is a lot more one-dimensional than Arceus-Water and therefore cannot find a place on various builds easily; it also experiences its fair share of downsides such as any factor detracting from its Multiscale leading to the obligatory devotion to hazards-control and avoidance of burns such as from Primal Groudon and Ho-Oh. I also find it to be pretty vulnerable paralyzed as it can't outpace to return to 100% via Roost as it can otherwise. Tldr Multiscale is a double-edged sword and Arceus-Water is a lot more versatile so Arceus-Water > Lugia.
Mega Gengar being the number one A-rank and Darkrai number two should be pretty obvious though in my opinion I wonder if others would disagree with me on this? Essentially, the former can counter Arceus and even Xerneas if you EV it correctly, can run a myriad of sets to fill out the team's needs meaning it is very versatile and hence viable, it has arguably the best ability in the entire game, and Darkrai falls just short of it in ranking while simultaneously sitting above the two other defensive Pokémon because although it is really one-dimensional, it fits well on both offense and balance, performs extremely well on hyperoffensive teams thanks to the fast Dark Void, and come on it's Darkrai one of the strongest Pokémon to come into existance plus no more Steel-resist and it can run shit like Thunder to get by certain checks at the cost of really only being able to bop Xerneas reliably to be honest.