Personally, I don't think Raikou really fits in A, A- seems perfect for it honestly. I always see it as a Pokemon which is all-around very solid but always seems to lack something to make it truly stand out. Pretty much all of its set have flaws to the point where none of them are really worthy of A.
Specs is generally considered the best set, and I think it's pretty easy to see how being locked into Volt Switch or Thunderbolt can be a problem when bulky Grounds like Hippowdon and Garchomp are this common, as well as Landorus which is not something you want to give a free turn to. HP Ice is also a pretty poor move to lock yourself into because of its poor BP leaving a lot of sweepers like Volcarona, Bisharp and Gyarados to use it as an opportunity to set up. Specs Raikou is neat but relies a lot on prediction from the user's part.
AV is a decent set, but really lacks in firepower. To give you an idea, it fails to OHKO offensive Garchomp and can even fail to OHKO Landorus from full. This makes it pretty poor against bulkier teams.
On the other hand, you have CM which helps a lot against fatter teams and which I honestly find better than AV, but struggles to pull its weight against offense since it lacks both firepower and the ability to eat up powerful special moves (though evading Bisharp is pretty nice).
LO sets are viable, but Raikou really does not like getting worn down and is generally much better at abusing Volt Switch to wear down its checks (which it can't really do with a Life Orb equipped) than sitting there and spamming attacks like Thundurus does, mainly due to its poor coverage
And then you have Zap Plate, which is actually pretty good, but suffers the same problem as AV : lack of power and inability to do much against bulkier teams.
And yes, you can argue that its versatility and the combination of its sets merits A rank, but it's still pretty limited in terms of what it can beat even with its multiple sets. Sure, there are a few things Sub CM Raikou can break past which it otherwise struggles against like Hetaran and Clefable, but there are still a bunch of common things it'll have trouble with no matter what. Hippowdon only really fears Specs HP Ice, Ferro can handle even Sub CM sets well (though it does run out of Gyro Ball PP in the process if it's not running Power Whip). XZard beats it, Altaria is a great check, Excadrill destroys it, MLatias walls it, TTar can chip half its health off by trapping it, Kyu-B beats it, Mamoswine beats it. There's just a lot of stuff it has to contend with and is both easier to handle and provides weaker support than the likes of Starmie, Tornadus-T and yes, MManectric.
Speaking of Raikou vs Mega Manectric, I really don't think it's quite as good as it even when you take into account the opportunity cost. MMan's advantages over Raikou are pretty significant and anyone who has used both Pokemon knows this.
Firstly, its speed enables it to outspeed a bunch more threats, not only Talonflame (which is a pretty massive deal, let's be honest) but also Tornadus-T, Scarf TTar, Adamant MDos at +1, Adamant MAltaria at +1 (though only relevant if it's weakened) tying with Lopunny, and being guaranteed to outpace Starmie.
It also has a much more useful ability which enables it to switch in on a good number of Pokemon. It's often said that Raikou is a specially defensive MManectric. But Raikou actually gets far fewer opportunities to make use of its special bulk, because many special attackers are either able to attack it physically (Latios, Keldeo, MDiancie, Starmie MGardevoir, Thundurus sometimes) or beat it anyway (MAltaria, MVenusaur, Dragalge, Volcarona for the most part) So you're left with Water types like Manaphy and Tenatcruel which MMan handles quite decently regardless, alongside stuff like MMetagross, Talonflame, Azumarill, Scizor, and Gyarados which it checks significantly better than Raikou does.
Manectric also has access to much better coverage than Raikou. While Shadow Ball and ESens are okay for Latis and Venusaur, HP Ice hits the former quite had already, and the latter is worn down by repeated Volt Switches especially when paired with entry hazard damage. And Raikou would dream of being able to hit Ferrothorn, Scizor, Excadrill and Metagross much harder.
But most of all, unlike Raikou, it doesn't have to make compromises. MMan already comes with solid power, no choice lock, and no recoil. It's not versatile at all, but it doesn't need to be, it already almost does more for a team in one set than Raikou is able to with all its sets combined. MMan is overall less constricting and more convenient to use than Raikou, to the point where, even when considering the cost of mega slot, it deserves to be one rank above it imo.
To add to the Infernape discussion, the set I've been using is SD, Close Combat, Flare Blitz and Gunk Shot paired with Scarf TTar. If you have a way to handle Slowbro and Hippowdon, it's actually pretty effective, Infernape's typing lets it set up more easily than you'd expect. I've also tried the same set but with Grass Knot over Gunk Shot which swaps Slowbro and Hippowdon with Altaria and Dragonite as checks. Needless to say, there's a huge amount of freedom in what Infernape can run. Nothing stops you, for instance, from running Close Combat+Grass Knot+Gunk Shot+SD : after all, you're hitting most of the tier with that, luring both fat waters and fairies, and giving it the ability to sweep lategame all at once. B- doesn't seem like overselling this thing at all : it's wonderfully versatile and has a lot of untapped potential.
Chomp was denied a move up to A+ recently but if there's a time when it deserves to move up, it's now. Chomp's ability to emergency check most physical attackers is a fantastic buffer for a lot of teams, especially given the huge number of teams right now that rely mainly on chip damage and how much Garchomp contributes to that. And sure, it can get overwhelmed by physical spam cores, but it's still putting a huge dent in them and shouldn't really be expected to take on the entire physical side of a team on its own, every defensive Pokemon is prone to that and at least Garchomp forces the opposing Pokemon to go down in the process. And that's ignoring other good sets such as Lum Berry SD, Scarf, and Sub SD Salac (very underrated and cool). It's one of the most defining Pokemon in the tier and deserves to move up.
