A little more reasoning on
and
:
I'm not going to bother explaining why Garchomp is good; everyone knows it's a great 'Mon and why it's the most common. However, even with the positives Garchomp have and how easy it is to fit it on a team, it isn't quite up to par with the rest of S Rank. If you look at S rank, everything is extremely threatening and have multiple sets that are just as viable as the most common one. Altaria can run multiple different variants of Dragon Dance; Earthquake, Refresh/Heal Bell, Fire Blast, and Cotton Guard. Although the latter two are very matchup reliant, the first two are extremely threatening and dictate the metagame quite a lot. Every team needs a way to handle Altaria. Garchomp, on the other hand, isn't the same. Garchomp is a very threatening defensive threat; it can be quite a nuisance to some popular Mega Evolutions, such as Mega Scizor and Mega Lopunny, but it is a 'Mon that will get worn down fast and can be over pressured due to team support. Garchomp's more offensive sets are pretty underrated, but they don't dictate teambuilding the same way Altaria does. Life Orb Garchomp is a very potent threat, but it doesn't provide the defensive utility Rocky Helmet Garchomp does. Salac Garchomp is very matchup reliant; it will get a chance to sweep or it won't based off of team preview. Some teams just don't give away free turns to Garchomp, which prevent it from setting up. After that there aren't any Garchomp sets that have a niche over some other 'Mons. It has a lot of utility, but it isn't a surefire counter to any 'Mon it's used for. Mega Lopunny commonly runs Ice Punch to get past it, Scizor is capable of wearing it down while also dictating the use of Fire Blast, and Mega Metagross can easily run Ice Punch to handle it.
Tank Garchomp is far and beyond the easiest Pokemon to slap on a team. However, not every Pokemon is amazing just because it's easy to fit on a team. Latios was the most used Pokemon in all of WCoP, but it isn't S worthy for a multitude of reasons. Garchomp is similar, in the fact that it is a great Pokemon with a lot of reasons to be used, but lacking the reasoning to be S.
When Landorus was around, Mega Latias had a lot of utility; not only as a simple check, but being a very annoying Pokemon with a lot of defensive utility. The biggest reason to use it has been gone for a few months, but it still has plenty of reasons to be used. It's not quite as good as the threats in A- or A, although it has plenty of niche. The CM Stored Power set is overrated and even if "unprepared teams" are weak to it, it takes a lot to pull off successful sweep. It can sweep, but it's a lot easier to do so on paper, especially in a high level game.
Anyway, in regards to the current slate:
D -> C
After the Landorus ban, people joked about using Nidoking as a replacement. Nidoking, even if it was just a joke at first, is a legitimate threat in OU. Not many teams are capable of handling it efficiently, as people will only worry about handling some physical Ground types, such as Garchomp, Landorus-Therian, and Excadrill, when picking how to handle Ground types in general. Nidoking also has some great coverage options, as well as Sucker Punch to scare out Latios and Latias. Nidoking also has a huge niche in the fact it absolutely ravages Clefable, even at +1. Nidoking can take on any Clefable set reliably and beat it as long as Nidoking isn't too low or if the Clefable has Ice Beam. Nidoking saw some good use in WCoP, and proved itself to be a big threat (see:
1,
2, &
3). I find it hard to believe anything in D or C- is better than it.
A -> A+
Yeah, this thing is insanely common, to the point where a lot of teams run ways to lure this or just dictate the use of a Magnezone. Even with all of this, Ferrothorn is still an insanely good 'Mon. The amount of support Ferrothorn provides to a team is huge; it checks Altaria, Gardevoir, Azumarill, Latios, Mega Gyarados, Manaphy, and Weavile. Although all of them, aside from Gyarados, could potentially hit Ferrothorn with a Super Effective move, all are still forced out by Ferrothorn. Ferrothorn may be a defensive 'Mon, but it takes advantages of free turns so much. No team enjoys giving away free turns to a 'Mon that can abuse them by stacking layers and and throwing out multiple Leech Seeds. Mega Sableye and Magnezone the only 'Mons that neutralize Ferrothorn and don't mind giving it any free turns. Ferrothorn definitely has its issues, but I don't think they outshine its positives.