I've got a couple of nominations, a bit controversial perhaps so no h8 plz.
Beedrill down to B+: I've been playing with beedrill quite a lot and in comparison to other mons in A, I just don't think it fits the bill. Firstly, to this support of its placement: "it's the best momentum grabber in the tier", I would say it would be more accurate to say that it simply has the strongest u-turn in the tier (by far, indeed). "Momentum grabber" to me implies a pivoting role which beedrill most certainly is not with its minimal bulk and zero defensive utility coupled with its heavy hazards weakness. In fact, even as a momentum grabber in its most basic sense of forcing something out and getting some free damage and a switch, it is still heavily impaired by two factors, namely its reliance on protect before mega-evolving and the fact that u-turn isn't able to scare much out, only non scarfed dark types without sucker punch, grass types and normal alakazam which, is about 6 fairly commonly used pokemon in UU.
As for its role as a sweeper, there are a ton of things holding beedrill back. Firstly, it is out and revenge killed by what are probably the two best megas in the tier at the moment (aero and zam) the former of which can switch in to any of its moves and the latter of which can actually set up a sweep by tracing adaptability if it manages to come in on it. It is also beaten by the most common scarfers in the tier (though, in fairness, it isn't guilty of this vice as much as mega zam who also packs a u-turn weakness) and pretty much anything with priority. But more problematic is its inability to do anything to something with a high defense stat as well as things that can handle its rather poor coverage and have some recovery other than u-turn which is not a satisfactory solution in my view. This puts another notch below things like megadactyl which can use taunt or adapt its coverage quite well for its team's needs. There are a lot of these pokemon that are viable several common. Here's a list: crocune, crobat, gligar, hippowdon, mega aggron, bulkydactyl, mega luvdisc (all of which nearly always carry recovery apart from mega aggron).
On top of this, it requires a lot of support including hazards on the opponents side and hazards off its own because all its counters are less hazard weak than itself. Which forces you to run rapid spin without megatoise leaving you with donphan, forretress, starmie, or xatu (the former two aren't great choices as they are easy defog opportunities for the opponent).
So to summarize my rambly rant, no defensive utility at all, very predictable move set including a reliance on protect, a lot of common counters that teams will usually be packing anyway, inability to break walls and sweep and its price as your mega slot really mark beedrill as lower than A ranked in my opinion.
My second nomination has been mentioned by others but I don't recall it being reacted to and that is: Crobat to S it honestly has so much utility at the moment, checking more pokemon in the tier than I think anything else which is still able to carry recovery and with multiple utility options to boot on the same set as well as good coverage with one move and ability to run 3 sets effectively, defog, choice band, and stallbreaker toxic, the latter of which shines really well in a tier with things like crocune and subcoil zygarde running rampant. It really fits the S rank description "Reserved for Pokemon that are almost flawless in the UnderUsed metagame. These Pokemon are usually able to perform a variety of roles effectively, or can just do one extremely well. Their use has low risk involved and almost always brings a high reward. Pokemon in this rank have very few flaws that are more than made up for by numerous positive traits."
Beedrill down to B+: I've been playing with beedrill quite a lot and in comparison to other mons in A, I just don't think it fits the bill. Firstly, to this support of its placement: "it's the best momentum grabber in the tier", I would say it would be more accurate to say that it simply has the strongest u-turn in the tier (by far, indeed). "Momentum grabber" to me implies a pivoting role which beedrill most certainly is not with its minimal bulk and zero defensive utility coupled with its heavy hazards weakness. In fact, even as a momentum grabber in its most basic sense of forcing something out and getting some free damage and a switch, it is still heavily impaired by two factors, namely its reliance on protect before mega-evolving and the fact that u-turn isn't able to scare much out, only non scarfed dark types without sucker punch, grass types and normal alakazam which, is about 6 fairly commonly used pokemon in UU.
As for its role as a sweeper, there are a ton of things holding beedrill back. Firstly, it is out and revenge killed by what are probably the two best megas in the tier at the moment (aero and zam) the former of which can switch in to any of its moves and the latter of which can actually set up a sweep by tracing adaptability if it manages to come in on it. It is also beaten by the most common scarfers in the tier (though, in fairness, it isn't guilty of this vice as much as mega zam who also packs a u-turn weakness) and pretty much anything with priority. But more problematic is its inability to do anything to something with a high defense stat as well as things that can handle its rather poor coverage and have some recovery other than u-turn which is not a satisfactory solution in my view. This puts another notch below things like megadactyl which can use taunt or adapt its coverage quite well for its team's needs. There are a lot of these pokemon that are viable several common. Here's a list: crocune, crobat, gligar, hippowdon, mega aggron, bulkydactyl, mega luvdisc (all of which nearly always carry recovery apart from mega aggron).
On top of this, it requires a lot of support including hazards on the opponents side and hazards off its own because all its counters are less hazard weak than itself. Which forces you to run rapid spin without megatoise leaving you with donphan, forretress, starmie, or xatu (the former two aren't great choices as they are easy defog opportunities for the opponent).
So to summarize my rambly rant, no defensive utility at all, very predictable move set including a reliance on protect, a lot of common counters that teams will usually be packing anyway, inability to break walls and sweep and its price as your mega slot really mark beedrill as lower than A ranked in my opinion.
My second nomination has been mentioned by others but I don't recall it being reacted to and that is: Crobat to S it honestly has so much utility at the moment, checking more pokemon in the tier than I think anything else which is still able to carry recovery and with multiple utility options to boot on the same set as well as good coverage with one move and ability to run 3 sets effectively, defog, choice band, and stallbreaker toxic, the latter of which shines really well in a tier with things like crocune and subcoil zygarde running rampant. It really fits the S rank description "Reserved for Pokemon that are almost flawless in the UnderUsed metagame. These Pokemon are usually able to perform a variety of roles effectively, or can just do one extremely well. Their use has low risk involved and almost always brings a high reward. Pokemon in this rank have very few flaws that are more than made up for by numerous positive traits."
