Breloom from the highest A to one rank lower.
If you look at the top of A rank, one could argue that it's closer to an extension of S rank than it is simply a slightly better grouping than the mid-high A rank Pokemon. Let me dissect the grouping just to prove this point:
Jirachi is given shit by a lot of people because some say it doesn't do much, but really it's one of the most 'viable' Pokemon out there. It saw 33% usage this season of ST, so far, and 37% last season, which says a lot as is, but I will go beyond just the fact that it gets a lot of use and explain why it's actually useful on the teams that elect to use it. Jirachi can use a plethora of sets - scarf, mixed SR, mixed lefties/EB, SDef, sub CM, CM wish, sub tox, etc. -- while none of these sets are even borderline "broken" or incredibly threatening, which is why people like me hold back from calling Jirachi S rank, they're all fairly good and collectively, they make Jirachi a comfortable fit on many teams and a versatile Pokemon to face and use. Often times, I slap Jirachi on thanks to sheer convenience - it can work as a check to Latis, Alakazam, Reuniclus, and a fair amount of other Pokemon...it can fulfill plenty of roles, so if you need a scarf, a SR setter, a lure, or a specially defensive pivot then you can use it and that's not to mention the fact that on Rain it runs CM sets that many teams are flat out unprepared for due to poor prep for it on a lot of bulky offensive and balanced builds. Overall, Jirachi is one of the best Pokemon in the tier when looked at as a whole and there really shouldn't be any questions as to why it's atop A rank.
Alakazam is seen as the premier cleaner on the generic sand spikes build that pretty much is seen as "standard" in the modern day. It's hard to check with confidence unless you pack one of a handful of Pokemon (SDef Jirachi and bulky Scizor being the first two that come to mind, but I suppose Chople Tar, the rare Sableye, bulky Slowking, and a handful of others can work). Honestly, Sash Alakazam is so unique in the scheme of things because it poses that consistent threat to teams and given that every major check to it is Spike bait for the most part, it's hard to work around Alakazam without breaking a sweat and perhaps becoming very vulnerable. I guess some people are also starting to use LO Zam ever since BW Cup finals where it did well (I think MDragon used it, but I'm not positive on that) and I've seen it a handful of times since, but that's not the main argument for Alakazam being so solid and worthy of being ranked near the top of A. The argument is with the sash set and how threatening it is given the context of the metagame and teams it finds itself on. Although I don't find Alakazam to be as useful or consistent as something like Jirachi, I find the added threat that it possesses to make it comfortably fit into the highest A rank.
Landorus-T is one of the best Pokemon out there right now and this shouldn't really be put in question by anyone. It has seen 34-35% usage in the last two seasons of ST and that's because it is incredibly useful. The Scarf set is quick enough to revenge kill almost anything that isn't at +2 speed or a faster scarf, the bulky set is a great defensive pivot that provides SR, and the double dance set is actually underrated, imo, but it requires support. Intimidate is so big as it weakens all the physical attackers that LandoT often pivots into and it allows LandoT to pose as a soft offensive check to Terrakion, Tyranitar, Scizor, Garchomp, etc. Scarf LandoT and Scarf Chomp are slapped onto the teams of a majority of BW players a fair amount of the time and that's because both are so good, but to hone in on LandoT, it is immune to ground, resists fighting, can control momentum, cripple Glisc/LandoT/Chomp w a well timed HP Ice, and it's strong w STAB EQ as well. It's just so good and so convenient, to sum it up. I think LandoT is among the top 5-7 Pokemon in the tier and it, without a doubt, belongs at the top of A.
Politoed is a pretty mediocre pokemon, but I think the fact that it is mandatory on every rain team and actually adapted into making some use of that role warrants it being a staple in the top of A as long as rain is as strong an option as it is right now (a lot of people say Rain is mediocre, but I find it to be fairly good right now, even if it's nowhere near as consistent/fall-back as Sand, it definitely can be used reliably. Politoed in itself is an ok last-ditch scarfer on rain, but the trendy sets are the offensive sets like Water gem, RestoChesto, Sitrus, etc. and Specs, imo, while defensive variants have always been ok on fatter builds. Overall, I just think the fact that Politoed has Drizzle makes it earn this spot, but I guess the argument more is "how viable is rain" and that really varies because many people frequent rain while some hardly use it. I just find it hard to move it any lower given the fundamental, necessary role it plays.
Latios is also at the top of A rank and any argument for it to go any lower is pretty laughable. Offensively, it's hard to actually counter and a majority of the "counterplay' is pretty much trapping it w/ Tyranitar. I personally use Draco/Surf/HP Fire/Roost on LO Latios the most and it really only struggles if it faces a healthy SDef Jirachi, Chansey, or, of course, Tyranitar, who limits it in general and makes Latios a very 1-and-done type pokemon, if even that, because a healthy Tar can switch-in to Latios. Psyshock on the LO variant is more common than what I use, but all that really does is make you less reliant on Draco and more vulnerable to a handful of pokemon depending on if you use Surf or HP Fire as the last move and then a lot of people use Specs, which hits like a truck. Defensively, Latios can work as a water resist, Keld check, electric resist, etc., so it's super convenient to use, as well. Latios is one of the most common pokemon (27% in tour 21 and 29% in tour 22) and it's a psuedo-staple on a lot of standard builds, so it being ranked here shouldn't be any surprise and shouldn't be questioned, either.
All of the above pokemon are either among the most (practically) threatening pokemon in the tier (Latios, Alakazam, and even Lando-T), the most useful pokemon in the tier (Jirachi and LandoT), or Politoed, which is a bit of a unique case, but I touched on it when I explained it. With all due respect to Breloom, I don't think it fits in with this group.
Breloom is incredibly threatening as it gets a Spore off w/ BW sleep mechanics and it does a fuckton of damage, but it's nowhere near as easy to use or as prevalent as any of these. It has seen roughly 15% usage in SR 21/22, which is a fair amount, but if you look at SPL usage stats, which are admittedly a smaller sample size, it only sees 5% usage while LandoT is at 49%, Jirachi is at 26%, Latios is at 24%, Politoed is at 24%, and Alakazam is at 13%. I don't think this necessarily says Breloom, when being used, is not as good as we think it is, but I do think that it says that building a high level team with Breloom is challenging because it provides minimal defensive presence when using the Technician variants while the bulkier Poison Heal variants are a rarity.
I personally enjoy using TechniLoom - hell, I even used it on a recent team that I used in SmogTour and BWLT playoffs...my favorite variant is LO w/ Spore, Mach, BSeed, and FocusPunch or Superpower - but the lack of defensive presence and lack of defined role it would play on a team strains teambuilding. Breloom's incredibly annoying to play around because you have to fodder a pokemon to sleep and then it puts a dent into another pokemon, but it often struggles to get in multiple times over the course of a game. The thing is that TechniLoom often barely only gets in once against offense, if even that, so it's limited there and its primary function is to be a bit of a breaker to bulky-o/balanced, when it can find its way in on a slower pokemon often enough, that lacks Amoonguss. One could argue that Hydreigon and KyuB also function as breakers of those type of teams that struggle against offense, but I think Loom is still a sub-rank above them solely because it provides sleep and priority can be useful, even if it's otherwise slower and much frailer than those two. The overall comparison of Breloom to Hydre and KyuB isn't the greatest because they're totally different pokemon in just about every regard, but the practical application and strengths parallel to a great extent, which is why I use it. The fact of the matter is that Breloom simply isn't as dominant as any of the top A rank threats, due to all of the practical shortcomings it has, and belongs in the tier just below them.
Volcarona from the second highest A to one rank lower.
I mean Volc is still one proper moveset and one rapid spin away from sweeping a fair amount of teams, but I feel like it's trending downward for a few reasons. First and foremost, Scarf Chomp is at its peak. People like BKC use it on practically every team and even people like myself, who tend to favor Scarf LandoT, have been using Scarf Chomp more and more as of late. In addition to this, the massive support Volcarona requires becomes less and less appealing to players as the tier hits a point of semi-stagnation, in my opinion, because we don't want to be tied down with a spinner, some means of assuring that spinblocking isn't easy or hazard setting in the first place isn't as easy as normal, and we don't want to be tied down to essentially having a predetermined win condition such a significant portion of the time in a game where there are so many variables and circumstances that arise. Volcarona is not very useful when it isn't sweeping and it can even be seen as a liability due to the hazard vulnerability. Volcarona still is an incredibly unique pokemon and the threat it poses with a QD against an unprepared team - it's pretty much lights out - is something that made me contemplate if proposing this was worthwhile because Volc can be so damn good, but really I just find it to be so demanding of support and inconvenient in the context of the tier that it should drop. Skarmory, Ferrothorn, Starmie, Scizor, Heatran, and, to a lesser extent, Reuniclus all are consistent pokemon that find their ways onto many teams with distinguished roles and all of them can be grouped into the top 15-20 pokemon that are often used and important to teams' functions. Volc, on the contrary, is a do-or-die type pokemon that can be deadweight or it can win the game and I feel like as more time passes, the upside isn't as appealing and it should fall down a bit into a grouping of less reliable pokemon.
Latias from the third highest A to one rank lower.
Latias is a good pokemon, don't get me wrong, but I hardy ever see it in anymore. I feel like with generation 6 developing into a tier where Latios sees all the primary uses as an attacker and mega Latias seeing the defensive use, BW has split up similarly. Before, the tank Latias at least saw use every now and then, but now it's non-existent, in my eyes, and the bulkier variants are all that come into play. With all due respect to them, the amount of time bulky Latias finds its way onto teams is fairly small and it just doesn't occupy much practical use at the time being because of that. I can easily build myself a team with Latias and it can be a successful member, but it just doesn't happen very often because bulkier builds with a passive CM or support Latias aren't trending at all right now while more offensive builds that want an attacker that can check Keld/waters/electrics all use Latios. I guess time simply isn't treating Latias well and I believe it should drop from a sub-rank as its current rank has two of the better breakers in the tier, the best spinblocker, and a solid attacker, in Terrakion, while the subrank below it is filled with more situational, sporadically used pokemon and that's where Latias fits best.
Amoonguss from the lowest A to one rank higher.
I rarely ever used Amoonguss until like 6 months ago, but the more and more I used it and find it fitting into my bulky-offensive/balanced builds, the better I see it as an effective sponge and defensive support member. It provides sleep, which is incredibly valuable in the tier given BW mechanics, and it can check Keldeo, Politoed, Rotom-W, Breloom, etc. I mean currently, it's ranked w/ Celebi, who lacks much of a purpose whenever it's used bar the periodic BP or NP variant, Gengar, who is a threatening pokemon that never finds its way onto teams, Hippowdon, who isn't used much at all anymore simply due to it being sorta slow and passive, Mew, who I actually like a lot and will refrain from commenting on because I think it might also warrant a rise in the future, and Gastrodon, who is a solid pokemon, but doesn't really find its way onto many teams due to there being plenty of options above it that fulfill parts of its role that end up being all that are needed on specific teams. Amoonguss is one of the only defensive breloom and keldeo checks and it also has a solid defensive typing w/ Regen and the ability to sleep things, so, imo, it has enough going for it to push it up a notch. Not really too strong on this one, tho, because a lot of people overlook Amoonguss and it only fits onto a limited amount of teams.