And tbh I feel like the whole Viability Ranking should be updated because there are so many changes to make.
To update the VR I'd say you can just write the changes and you don't have to write an explanation for it because then way more ppl will participate in this. I realise it at my own, I'd change quite a lot things but I'm just too lazy to write that much and most things should be just obvious anyway (for example the aspect with Suicune and Tapu Fini). If there are very controversial changes (or if you want a Pokemon to be S because that is a "big step") you can ask and discuss but for most things the community should've the same opinion.
Yeah, I would like to ask everyone be patient with the Viability Rankings. We are a much smaller community than any of the tiers or VGC, yet we have the most Pokemon to deal with out of any metagame. So it's unavoidable that our rankings will move pretty slowly.
Anyway, here are my personal thoughts.
Garchomp: Drop down from S rank to A rank: Like others have already stated, Garchomp's effectiveness has just dropped significantly. Focus Sash, which I once considered the best and most reliable set in all of Battle Spot, is not so great anymore now that Stealth Rock is everywhere. And without that, it dies to most set-up sweepers rather than being a great check to them. Scarf sets are still really good though, but with both of its STABs having immunities, it can hurt your momentum if you're not careful. But I still think it's good as an offensive check to +1 Gyarados, +1 Zard X and Y, +1 Volcarona, etc.
Kangaskhan-Mega: Drop down to A- rank: Jhonethan basically said it all. "Damage is not what it used to be" being the key phrase here. Misses out on a lot of unfortunate KOs. With the Sucker Punch nerf along with the Parental Bond nerf, you are no longer afraid of just using a Scarfer to revenge kill it, as it has no way to boost Speed outside of getting passed it.
Mega Charizard X: Somewhere in A rank idk: Kind of ambivalent on this one, so I'm just bringing it up for discussion. I see a lot of people using Dragon Dance sets on it now which I don't think are very good. But there must be something wrong with SD Flame Charge if people are using Dragon Dance now. And its usage is slipping, Y is becoming more popular (which I predicted, and then changed my mind, and then it came true, so like fuck) I think it should still remain in A somewhere, but not sure where.
Mega Charizard Y: From A rank to A+ rank: This mon is so fucking good, now that people have finally realized I was right the whole time and that Flame Charge is actually a godlike set on this Pokemon. It can sweep entire teams now after just one turn of set-up, which can easily be achieved by sacrificing a Pokemon or using Hippowdon Yawn or a smart double switch. It and Volcarona are the reason that Stealth Rock is everywhere now. S-Rank if Stealth Rock didn't exist; it has a little trouble because Fire-types can decently switch into it and force it out a few times. Unlike Volcarona, it can't afford to use Roost and can't boost and kill Fire-types with Inferno Overdrive.
Azumarill: From A- rank to B- rank: Toss dis nigga. It is so bad. Sad for me to say, but it's true. I'm honestly not even sure what it does anymore. Its bulk is not that impressive anymore with all the power thrown around, and its so slow it has to take two hits before it can ever do anything. You are rarely ever able to set up Belly Drum, and even after that it's not so great because it can't OHKO things like Metagross, Kanga, or Gyarados with Aqua Jet. Not to mention Tapu Lele Psychic Terrain shuts it down completely. Really sad because it's one of my favorite Pokemon, but just objectively I think it is pretty bad. No way this thing is better than Hydreigon, Pheromosa, Gliscor, Gyarados, Nihilego, Serperior, or Tapu Bulu.
Cresselia: Maybe from A- rank to A rank: There's been a resurgence of Cresselia use lately, I guess that's just how these things come in cycles. I personally haven't used it much, so I'm leaving this as a maybe. With Trick Room becoming popular again as a way to counter all the insanely fast sweepers, it has been finding a niche again.
Rotoms: Drop from A-Rank to B or B+ rank: Everything's been said already. They don't cut it anymore. Although I would say I find Rotom-H a little better than Rotom-W, so perhaps keep Rotom-H in A- and drop Rotom-W to B.
Nihilego: Up from B+ rank to A- or A rank: It's funny that a Pokemon considered to be useless at the beginning of the generation would give Celesteela competition as the best Ultra Beast. It's been an amazing anti-meta threat lately, thanks to its amazingly convenient 103 speed and great special defense. It takes out Charizard, Garchomp, Tapu Koko, Tapu Lele, Tapu Fini, Tapu Bulu, and Thundurus-T. It's being used on a lot of successful teams now. In return for having horrid physical defense, it is a Choice Scarf Pokemon that can actually switch into the tier's strongest special attackers and then kill them.
Alolan Marowak: Drop from B+ to B (or B- if we wanna make it): Despite my best efforts, I cannot find a way to justify using Marowak on most teams now. However, I don't think it's totally useless. It can still work on Trick Room teams. Generally, Mawile is much better, but Marowak does have a better time against Steel-types than Mawile. Unfortunately, that means there is a big stacked Ground weakness on your team, and Marowak is easy to wear down. It's also a decent switch-in to Mega Charizard Y, but I think Heatran and Tyranitar are generally better in that department and offer more for your team.
Xurkitree: Drop from B rank to B- (or C+ if we wanna make it): What can I say? Another one of my favorite mons that doesn't cut it anymore. Scarf sets are outsped by every other Scarf set, Hypnosis sets use up your Z-Move and are outsped by every Scarf set. Unlike most of the threatening sweepers, it can never boost its speed twice, so it's easily revenged. Its lack of a second type lets it down hard. It is outclassed by Thundurus-T in almost every single way. 101 speed is just too good to pass up, and the special attack difference is negligible, not to mention the fact it can use All-Out Pummeling. But the biggest problem is that it cannot switch in to anything, due to its pretty bad typing. Thundurus-T gets the valuable immunities to Ground and Electric which let it easily switch in to things. Xurkitree has no such privilege. Still, I think it should stay on the rankings because Z-Hypnosis can actually be quite devastating if you lead with it, since that's the point where momentum is most neutral.
Those are all my opinions for now! First time making a viability ranking post so my apologies if my rhetoric and movements are a little too extreme.