Mega Metagross (S > A+): Agree. While I do think Mega Metagross stands out in comparison to all the other A+ rank Pokemon, I also think it really has no business in S ATM. Mega Metagross is very powerful; Mega Metagross is a threat. HOWEVER, like most people have been saying, Mega Meta is something lots of people prepare for now (Mega Scizors everywhere), and it isn't as diverse as other S rank pokemon.
Ferrothorn (A > A+): Disagree. Ferrothorn is an excellent bulk/support Pokemon; it has access to Leech Seed, Spikes/SR, ThunderWave, and Iron Barbs is a very nice ability. The thing is: Ferrothorn lacks a reliable form of recovery, and it's weak to 2 very common (and very good) offensive types. I've seen people mention "why is Ferro A+ material but Skarmory isn't?", and I 100% agree with that.
Rotom-W (A > A-): Agree. Not much to say here really... Rotom-W is a really nice bulky water and a great Birdspam counter, but the thing is definitely prepared for by now. People are running enough shit to handle Rotom-W. Also, it really only has the 1 physically defensive set; Scarf and specially defensive are pretty ass lol.
Celebi (A- > B+): Disagree. Celebi is a great support Pokemon. It has enough bulk to run a physically bulky set, a specially bulky set, or a mixed bulk set, it has a great ability, it has nice typing, and it has an excellent support movepool. Celebi can support more offensive teams without hurting momentum too much thanks to SD/CM/Sub/Nasty Pass or Healing Wish, it can spread para with TWave, it can provide SR support, it can force out set up sweepers with Perish Song, it can provide cleric support with Heal Bell (and Leech Seed sorta), etc. Also, you can run moves like Earth Power/HP Fire for a more offensive Baton Pass.
Reuniclus (B+ > B): Disagree. Reuniclus is a really great fat Psychic. Its CM set is probably its main one, but it also has offensive Trick Room, Specs + Future Sight, more offensive LO options, as well as some other options like Colbur Berry and Regen Pivot (these aren't that great tho lol). On the Trick Room set: I've used this set on NON-TR teams a couple times. It's very nice on balanced/bulky offense builds (AKA what Reuni works really nicely on), and it's a nice way of tripping up opposing offense teams.
Feraligatr (B > B+): Agree. Gatr is a nice alternative to Azumarill, but it's also a bit more than that. Gatr's SD + Aqua Jet set isn't too good IMO, but this is where I feel Feraligatr can be a nice alternative to BD Azumarill. Feraligatr's Dragon Dance set is what sets him apart from Azu IMO. DDance allows Gatr to easily wallbreak, and it isn't total ass against offense.
Keldeo (A+ > S): Disagree. Keldeo is a GREAT Pokemon, but I don't think it's worthy of S. While it's nice that Keldeo checks Weavile (which is obviously getting more and more popular), Bisharp, and all that other stuff, that isn't much of a reason for it to rise. It's still rather predictable; it has a shallow movepool and like 3 popular sets (Specs, Scarf, SubCM). I feel like it has the potential to be S (again) in the near future, but that's only if 1.) we see even MORE of a rise in usage of the things it checks and 2.) we see sets like CM + 3 attacks, Mystic Water/ EBelt, and more rise in popularity.
Dragalge (B+ > B): Disagree. Dragalge has some nice bulky offensive sets in Draco Plate/Specs, but it also has nice bulky/support sets. IMO Poison + Dragon is really nice typing defensively, and Dragalge further takes advantage of this by not being passive as hell (thanks Adaptability), and with Toxic Spikes. There are other TSpikes users out there, but Dragalge is one of the better ones because of its combination of solid bulk + solid defensive typing + ability to hit back + Scald.
Magneton (C+ > B-): Disagree. The only thing Magneton has > Magnezone is the ability to outspeed shit like Torn-T and Weavile (and other shit I'm forgetting ATM) when Scarfed. While this is nice, in most cases, that extra bit of bulk and power that Zone provides is much more desirable. Magneton has a nice niche, but that niche + Zone rising isn't enough for it to rise.
Gengar (A+ > A): Disagree. Even with Weavile and Torn-T becoming more common and Pursuit users running around, I think Gengar is still a really threatening and diverse Pokemon that's worthy of A+. Not much really walls the thing (outside of shit like AV Torn-T and Mega Scizor which are rising), it has excellent offensive coverage, and it can run some cool sets like stallbreaker, Hex, Sash D-Bond, Scarf, etc. While it is true that stuff that beats Gengar like Mega Scizor, (AV) Torn-T, and Weavile are rising, that isn't enough for it to drop to A IMO.
Mega Ampharos (C > C+): Agree. Mega Ampharos has really nice defensive typing and stats, a nice support movepool, great offensive moves, and a great ability. NastyPass + Mega Ampharos or just plain agility Mega Amphy is a really nice breaker thanks to the aforementioned offensive movepool + Mold Breaker, but I feel like its more bulky/support options are overlooked. Amphy is fat, but like Dragalge, it isn't passive as balls. You can run the pretty neat RestTalk set, or you can go for some different options like Heal Bell/Discharge. Also, with Torn-T being all the rage nowadays, I feel like that gives these bulky Mega Ampharos more of a purpose than before, and with these lower viability Pokemon, a little niche like that can mean a lot :p
Hippowdon (A > A+): Agree. Hippowdon is such a HUGE Pokemon right now; the thing has a type of balance named after him (Hippo Balance). Hippowdon is the best bulky ground type IMO; while Lando-T and Bulky Garchomp can work a bit better on more offensive builds due to not being as fat + being more offensively capable, Hippo wins due to being the fat one lol. As we all know, this thing can run mixed bulk sets, specially bulky sets, and physically bulky sets (usually better on stall/fatter builds). It has great support moves like SR, Whirlwind/Roar, Toxic, it has a nice recovery option, it has Sand Stream for sand teams/cores, and it just plain handles lots of common threats like Char-X and Talonflame which is always nice. Not to mention the mixed bulk/specially defensive sets work really nicely on balance considering they aren't so fat that they totally kill momentum. I could go on here but eh... lots of other people are doing a better job of supporting this nom :p
Noivern (C- > D): Agree. lol why is this thing in C- i have literally only seen like 2
No comment on Serperior, Infernape, Tangrowth, Cobalion, Mega Aggron, Chesnaught, Tentacruel, Mega Gallade, and Mega Diancie because I haven't had enough experience with these to provide some decent opinions + I got lazy.