Gonna hop on the Mega-Metagross for A+ bandwagon. Meta-Metagross is the epitome of a very good, but not consistently great pokemon in the ORAS OU metagame. While other S rank pokemon like Mega Charizard-X or Mega Altaria threaten a majority of teams with their sweeping capabilities (not to mention each of these has another solid variant outside of DD builds, being bulky wisp and mixed respectively), Mega Metagross isn't as threatening and most teams are able to handle it quite easily due to the plethora of common, viable checks and counters the tier currently has. Add onto this the fact that with a boost or two, something like the aforementioned Mega Charizard-X or Mega Altaria has the ability to break through or cripple their counters with boosted attack and go onto sweep later in games often while Mega Metagross's main answer to common counterplay is Grass Knot, which fails to hit various common checks/counters like (Mega) Scizor, Skarmory, fat Chomp, Lando-T, and Gliscor (Ice Punch hits the last few, but Ice Punch + Grass Knot is quite uncommon and makes you compromise your overall coverage, so you're always missing out on a lot of things regardless of how you use it). Overall, Mega Metagross fits into the group of strong offensive threats that can be annoying to deal with at times, but almost always will have a counter or two and thus require support or the perfect moveset to do well (making them less consistent than top threats like Mega-Zard-X and Mega-Altaria). Given this, I think it fits more into
A+ than it does S.
Rotom-W has always been a poke that has seen a fair amount of usage on the ladder dating all the way back to BW1 (despite it being consistently lackluster in my opinion) and on bulky offensive and offensive teams in general as a bulky pivot, but I simply think that it doesn't warrant much usage in the current metagame unless you're being lazy in teambuilding or facing very specific circumstances that aren't often encountered. Obviously, the water-electric typing with levitate and a fair amount of natural bulk is great, but many teams rely on things like Hippowdon or other bulky grounds (think Garchomp or Landorus-T) to check scarf Landorus-T, Mega Meta (Rotom-w is a shitty check to Mega-Meta, too), offensive Scizor, Talonflame, tyranitar, etc. that Rotom-W generally is used for while speaking in terms of water types, Manaphy, Keldeo, Azumarill, Slowbro, Starmie, and (Mega) Gyarados all possess higher or comparable viability (almost all have higher) than Rotom-W while having some overlap in terms of niche with it and then other pros that Rotom-W lacks (Manaphy destroys balance, Keldeo is simply brilliant, Azumarill hits hard and can sweep with BDrum, Slowbro is a top tier physical wall with slack off + regen and sweeping capabilities, Starmie can spin, Mega Gyara can sweep, etc. - Rotom-W's "niche" over all of these is being "annoying" to deal with as defensively speaking, it isn't better than Slowbro or other things for the most part and this isn't much of a niche to boast about in comparison to others who are in A or A+ rank). I feel that most Rotom-W teams slap it on to "cover" a lot of things despite their being better or more optimal routes in teambuilding to take to cover these threats. Dropping it to
A- alongside other less consistently worthwhile and viable pokemon seems like the best way to go.
Celebi has sort of just been sitting there in A- for the last couple months (I think?) despite it not really getting much use at all, especially on higher level or tournament teams. The SDef SR variant is as unproductive and underwhelming as expected while the BP variants provide some useful support, but don't find their way onto many teams and simply don't warrant it to be A- alone. I've seen a couple Nasty Plot celebi, which can be cool and a measure against balanced teams if you think about it, but it's still quite easy to check/counter and easy to revenge kill on offense while you compromise a lot of bulk and you have trouble finding the right moveset on it, so I just find Celebi to be lackluster overall and not worth its spot in A- (trying to keep this brief since this post is going to be long, so tl;dr = Celebi for
B+).
Hippowdon is one of the most used pokemon in the current metagame - hell, it was the second most common poke in WCOP round 1 in ORAS OU, so it's obviously a prominent defensive presence and stealth rock setter in the tier. With that being said, it does face some competition from Rocky Helmet Garchomp and even bulky variants of Landorus-T, but on bulkier teams it tends to be the best fit for the role of ground type SR mon. There are so many teams with like Hippo + Clef + Slowbro cores and this archetype is anchored by Hippowdon while Hippo itself has plenty of other uses given its convenience and ability to wall a significant portion of the metagame. Given standard sets, the following pokemon ranked A- or higher are walled or at least checked by Hippowdon: Mega Zard-X, Mega Metagross, Bisharp, Mega Diancie, Excadrill, Garchomp, Heatran, Mega Lop, Talon, Tyranitar, Jirachi, Klefki, Magnezone, and Raikou - obviously, some of these can break Hippo with the right set or the right circumstances, but Hippowdon generally has the edge and in a realistic scenario, it can wall all of them. Given the above, the defensive presence and convenience that the standard mixed def Hippowdon grants a team is simply too much for it to remain in A rank, so I think that a rise to
A+ is warranted.
There was a point in later XY when Gengar was seen as quite fast and not many teams had a switch-in to it and it deserved to be in A+ despite being the glass cannon that it is, but with all of the fast things being common (Mega Meta, Mega Diancie, Mega Lop, Latios, Latias, Mega Manec, Thundurus, Talonflame, Tornadus-T, Alakazam, Mega Alakazam, Weavile, Mega Aero, Raikou, Serp, etc. all tie or outpace Gengar) in the current metagame and gengar being fairly hard to fit into a team as it's not a reliable fairy check and there are so many things to defensively cover right now, I simply don't see it fitting into A+ anymore despite the lack of true defensive counters to Gengar in the tier. As I said, I wanted to keep it brief, so I'll leave it at the metagame being quicker makes Gengar less viable and there is simply less room to fit it onto teams, so a small drop to
A couldn't hurt.