I actually disagree with Terrakion being moved to A-rank. Yes, it can revenge kill, yes it has a cool double dancing set, but it by no means is capable of sweeping entire teams. It can only do this if given extreme support (like SR + 3 layers of spikes + lots of phazing), but at that point virtually every mon (including those in lower tiers that are fast) can do that. Terrakion is famous for two sets, its double dancing one and its Choice Scarf one (it can do Sub + SD and CB too but it can't do the second one very well and the first one's usefulness has gone down with Roar Giratina-a being so popular). The Choice Scarf one is an example of a niche set, it occupies the niche of being able to revenge kill a few threats, namely weakened ExtremeKiller Arceus, Rayquaza, and the Kyurems. The Double Dancing set is kind of strong, but its very hard to setup due to the fact Terrakion just lacks resistances. Without Dual Screen or Wobbufett support, acquiring those two boosts is practically impossible. It doesn't help much either that its still stopped by Great Wall Giratina, I'd even go so far as to say its outclassed by Groudon (but it can beat Skarmory, and you can also do the whole I have two Double Dancers to weaken the other double dancers checks so that the other mon can sweep). Regardless, there are only specific situations where you'd use Terrakion over another mon (like you need a revenge killer that can stop Ekiller Arceus (it fulfills a niche)).
With this being said take a look back at the B-rank description.
Original Post said:
B Rank: Reserved for Pokemon who cannot sweep through or wall significant portions of the metagame, but can properly fulfill a given offensive/defensive niche. Support Pokemon in this category have flaws that prevent them from doing their job or are setup bait for dangerous sweepers. Pokemon who are partially outclassed by a Pokemon in A or S Rank, but are otherwise very dangerous, may also fall into this category.
Giratina-O, also, in my opinion should stay A rank. Giratina-O is a great spin blocker, but to call it the best might be a stretch. Each spin blocker has their own strengths. Giratina-O is the only one immune to hazards, and its the only one that has a decently strong Dragon Tail and can shuffle nicely, but it lacks recovery (outside of Rest), has a weakness to Dragon-type and Ice-type moves, and it can't sweep that well or wall well (it can stall break somewhat nicely I guess with Sub + CM or Sub + Dragon Tail), its a tank. Unlike all of the S-rank members, it can't turn around and stop its checks by surprising them with an uncommon set or move (for the record, this is why I think Ghost Arceus is S Rank).
It's easy to setup cm? At most you'll have 1 and at which point your opponent will have their cmceus check/counter.
I'm assuming CM Ghost Arceus's set is CM / Judgment / Focus Blast / Recover.
The only true CM Ghost Arceus counter is Chansey (this gets completely stomped on by Sub CM variants). Ho-Oh can come close I guess but if Stealth Rocks are up it shouldn't be happy... it can't OHKO back while it can just Calm Mind again and Judge the thing to death. Only physical Choice Scarf users pose a chance at stopping CM Ghost Arceus (well that or you can phaze it but Ghost hits pretty much every phazer barring Skarmory and Arceus forms super effectively), and the strongest of them, Choice Scarf Kyurem-Black, barely 2HKOes (176+ Atk Kyurem-B Outrage vs. 252 HP / 4 Def Arceus-Ghost: 216-255 (48.64 - 57.43%) -- 93.36% chance to 2HKO) and gets OHKOed in return by Focus Blast, you can try using Choice Scarf Zekrom instead, it can try to 2HKO with Bolt Strike (252 Atk Zekrom Bolt Strike vs. 252 HP / 4 Def Arceus-Ghost: 202-238 (45.49 - 53.6%) -- 38.67% chance to 2HKO) but chances are its move will get pp stalled instead. Other threats like Scarf Garchomp just flat out fail to 2HKO. If your using Scarf Ogre as your check Choice Scarf Water Spout doesn't even come close to OHKOing (252+ SpA Kyogre Water Spout vs. +1 252 HP / 0 SpD Arceus-Ghost in rain: 256-303 (57.65 - 68.24%) -- guaranteed 2HKO), while Ghost Arceus can use Judgment the turn after, weakening Kyogre to the point where it can Recover off the damage. Surf doesn't 2HKO after Calm Mind.
Point being said, Ghost Arceus is extremely hard to stop (unless your using Darkrai (<- this guy needs to have not used Dark Void yet)/ Chansey / Sp Def Kyogre (can only Roar, which is just delaying the threat and taking a hit, does stop it temporarily though) and this is without support. Pairing it with Dual Screen or Wobbufett support (or even just letting something else take the Dark Void from Darkrai) or some kind of double Calm Mind user thing makes it a LOT harder to stop.
This isn't even factoring how well the support set does against stopping threats while preventing spinning and how decently Ghost Arceus runs a SD set.
In my experience, Calm Mind Giratina-O is as good as Cam Mind Arceus-Ghost. Levitate allows it to avoid Toxic Spikes and Spikes; Toxic Spikes would ruin Arceus-Ghost's sweeping potential. Giratina-O can use Substitute which lets it set up on Chansey and Ferrothorn lacking Gyro Ball. Giratina-O uses Aura Sphere unlike Arceus-Ghost who has to rely on Focus Blast.
Calm Mind Giratina-O lacks Recover, and has a dragon-type weakness making it prone to being revenge killed by scarf dragons, this is where you draw the line. Both can run substitute to stop Chansey (although I will admit that Giratina-O is a little better of at doing this thanks to levitate (avoids Toxic Spikes).
In my experience, Giratina-O is more reliable than Arceus-Ghost for checking Extremekiller Arceus. Arceus-Ghost has to rely on Will-O-Wisp to beat Arceus-Normal. If Arceus-Normal holds a Lum Berry, the chance of hitting Will-O-Wisp twice is 56.25%. This means that Arceus-Ghost will fail to beat Arceush-Normal almost half the time. Physically defensive Giratina-O, unlike Arceus-Ghost, does not have to rely on Wil-O-Wisp to check Extremekiller Arceus, uses Dragon Tail which is 90% accurate. Dragon Tail in conjunction with Stealth Rock and Spikes means that Arceus will be at less then 25% health after being phazaed out and sent back in. With this remaining HP, Arceus is unable to set up and anythng on your team should be able to KO it
Firstly, Ghost Arceus gets Roar so its not like it can't phaze. Secondly, having less then 25% health is a pretty far jump 252+ Atk Griseous Orb Giratina-O Dragon Tail vs. 88 HP / 0 Def Arceus-Normal: 105-124 (26.05 - 30.76%) -- guaranteed 4HKO. With Stealth Rock that peaks at 54%. I know you mentioned Spikes but 2 layers of spikes + SR are required to get it below 25% (which honestly sounds a bit farfetched...). Keep in mind this ignores the possibility that Extremekiller is using Shadow Force (in which case WoW > Roar / Dragon Tail), and that if it is using Shadow Claw that its only a one time check.
See the first set there? Yeah, it is the Mixed Attacker which has Hidden Power Fire to roast those steel types in the sun. Next time you say something, make sure that it is valid.
Just wanted to add that it can also use Thunder and Aura Sphere to beat Steels too :P. Trick Room actually used mixed Giratina-o w/t Thunder in his Drown All team, which I think none will disagree with was extremely influential on the metagame at that time.