Kyurem-B has a massive Attack stat, an impressively usable SpA stat and fearsome bulk. His movepool may not be as fancy as Dragonite's but he has all the tools that he needs. Kyurem-B is very well known for being able to go Banded, SubKyurem, or Mixed. This alone places an advantage over the aforementioned Haxorus, as Haxorus is monotonous and straight-foward in function. Kyurem-B's offensive prowess in conjunction with his impressive bulk seriously allows him to threaten any team, offensive or stall. Just because he lacks Earthquake doesn't make him a lower-ranking Pokemon. He has Earth Power at his disposal which allows him to dent those Pokemon that you mentioned. His Dragon Tail is also pretty strong and while it won't KO a Steel Type, it will seriously wear it down with Hazards in tandem with a possibly Boosted Dragon Tail from Hone Claws. Kyurem-B also has possession of Tailwind. It's surprisingly dangerous and can make that Terrakion think twice about switching in. All in all, Kyurem-B definitely is A-Rank material. I'm kinda wondering if something with that much power and coverage will stay OU for long. If so, I want to see it rise to high-A or even S-Rank, cause when we talk about power, everything else, like his boring and small movepool, simply just hold little to no relevance as he will be smashing any Pokemon willing to talk in.
The fact that you said Infernape was useful in certain situtations just goes to show why he isn't S,A or B-Rank. His niche is very small and to make things worse, it's very situational. Being able to speed-tie with Terrakion isn't really a niche as well. And if it is it isn't very impressive as the other musketeers can do the same as well, not to mention the Lati twins. The SkarmBliss core still holds tried and true but it's been getting rarer and rarer to encounter. And once again, it's a small niche. Kyurem-B, Terrakion, Lucario and others can fulfil that same niche that Infernape holds. In fact, they can do that niche and more which allows them to outshine Infernape. Honestly, C-Rank is pretty solid for the Ape and unless something happens, Infernape isn't looking to be climbing up that ladder anytime soon.
The only thing I like Kyurem over Kyurem-B is the design haha
Otherwise, sadly, the ugly ass Kyurem-B outclasses Kyurem too much.
It can do pretty much everything Kyurem can ... just better and in a more efficiently powerful manner.
Infernape is not too good for A-Rank LOL
I've used him and I've played against him. He's alright I guess...But he seriously has the lost the flames that made him the DPP King of long ago. 104 Offense is not bad at all, but it's just not enough I feel to really hammer it out. Not too mention that his biggest STAB in Fire Moves are easily resisted by top threats like Latios, Dragonite, Latias, Jellicent, Heatran, Tyranitar and so forth. Close Combat is still devastating as ever I feel but then again coming from Base 104 isn't going to be punching down too many physical walls without some form of a boost whether it be a Dance or Drought. Infernape's biggest niche over any Pokemon would have to be Mixed. Everything else, sadly, is outclasses and honestly, his other sets should only be used when his unique typing or effectiveness on Drought is needed or something. Swords Dance is taken over by Terrakion and Lucario while Nasty Plot is just not powerful enough to compete with things like Thundurus-T.
Now his Mixed Set: It just isn't that strong enough. I've used it in the past and it seriously needs a heck of a lot of support. Frail, susceptible to all entry hazards, easily nerfed by Drizzle and not enough OOMPH in his stats leave him kinda sad. I'd say B-Rank is decent enough. But A-Rank is not a home for the Ape. Or S-Rank for that matter.
I think I'll pass. While I think A-Rank might be a little high for it, I understand the reasoning behind it. (I mean, I'd rather Hydreigon be in the same tier as it, but whatever, I've tried that already.) I think it's fine where it is.