After some deliberation amongst the VR council of everyone's posts, we've decided to make the following changes:
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S -> A+
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A- -> A
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C+ -> B-
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C+ -> B-
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B -> B+
The arguments for each rise and drop primarily follow from everyone's posts. You may notice that Muk-Alola has risen despite no formal nominations. The VR council has decided to give it a raise due to its strong showing in high-level play, being a highly useful component of defensive teams to take out problematic Ghost- and Psychic-types such as Lunala, Deoxys-A, and Mega Gengar that ordinarily pose a massive threat to such teams, while acting as a soft-check to offensive Dark- and Fairy-types. Poison Touch and tools such as Knock Off also limit Muk's passiveness against most targets save things like Mega Lucario.
There were some other noms that were not accepted. Here's some brief explanations detailing why:
Lunala - Lunala's presence is one of the main reasons why most teams need a Ghost resist nowadays, so it's metagame impact certainly shouldn't be ignored. However, Lunala is somewhat difficult to fit on a team due to its murky defensive synergy. While Shadow Shield enables it to survive a massive range of attacks, its lack of useful resistances inhibit its ability to actually switch into threats and it struggles to get the most out of its useful defensive properties (Normal/Fighting immunities) without using a Choice Scarf. The increase in Muk usage on defensive teams is also a detriment to Lunala's ability to break cores as it lacks solid coverage options for dealing with it. The sheer offensive threat Lunala poses is good enough to give it a spot in the A ranks, but its limited defensive synergy keeps it from going higher than A-.
Zygarde - Zygarde can run into some trouble transforming in practice due to Power Contruct's interaction with Leftovers, and its low offensive power and 4x Ice weakness can be exploited. I think this mon could potentially rise in the future, but I'd like to see some more usage of it in high-level play (no the SubCoil set is not reliable, as fun as it is) before considering it for a bump.
Tapu Koko - While Koko is a good offensive check to Kyogre, Ho-Oh, and Yveltal, it can't actually switch into any of these threats, limiting it more to the role of a revenge killer. Koko also faces extremely stiff competition with Xerneas (I don't think I need to explain how good Xerneas is) for that Fairy slot on offensive teams and struggles to break Electric-type resists on its own (though Nature's Madness is useful for PDon at least), and its offense becomes extremely limited once its Z-move is consumed.
Dugtrio - Dugtrio's niche is fairly useful, but it also confers essentially zero defensive synergy, is very weak once its Z-move is used, and has little versatility outside of its niche. It also tends to be weight against hyper offense teams as Dugtrio can't reliably pin down Primal Groudon if its packing Rock Polish. Dugtrio essentially requires a team built around abusing Arena Trap in order to justify it, which isn't really a quality of a B rank mon.
Landorus-I - While Landorus-I is probably better than most of the stuff in D rank, it doesn't offer too much defensive synergy other than a somewhat decent switch into Primal Groudon and can still be checked by some balanced/BO staples such as Giratina-O and Blissey, plus it is rather easy to revenge kill by Scarfers and faster offensive threats such as Mega Lucario and Deoxys-A. B rank is way too much of a jump from something that was firmly in D last gen - a more modest rank increase might be appropriate in the future if Landorus-I sees more high-level play.