I disagree with Darkrai for S. It is still very good imo, but just not on an S rank level. I, for one, have trouble fitting it on a team that isn't full offense, primarily due to the other archetypes needing more defensive niches in Pokemon than anything (this is where they can use Darkceus or Yveltal), rather than a frail mon that hard stops defog once, puts something to sleep once, and has some potential to sweep. Darkrai almost needs Life Orb to do its job properly, otherwise its power is extremely underwhelming. This is terrible in the versatility department, and although Arceus-Normal has almost no versatility either, it is litterally the best cleaner in the game due to natural bulk (easier time setting up) and being immune to almost all forms of revenge killing (which is another niche E-Killer has: revenge killing threats with Speed boosted). While Darkrai lost a handful of revenge killers, it gained a rather centralizing check in Primal Kyogre who lives any +2 hit and KOes Darkrai with Origin Pulse or Scald after something has been slept. Ho-Oh is better than ever also, and is still a very nasty Darkrai check and Sleep absorber.
Darkrai is something that only faster offensive teams can utilize fully a lot of the time, it has limited versatility, is easier to check than the S Rank mons after Dark Void is used, and has few defensive niches to offer. Although Darkrai is a strong mon on hyper offense and archetypes like it, it is limited outside of this and should therefor not be S rank.