Okay, I'm going to make a nomination that's probably going to be controversial, and I'm not entirely comfortable with it myself, but I think Ferrothorn should be dropped down to A.
Now, let me make this clear : Ferrothorn is a great wall. It has a fantastic typing, a very useful ability, it deals with a ton of stuff, has really good utility, and fits on a lot of teams very well. However, there is one big problem with Ferrothorn that no other A+ rank really shares, and which bothers me a lot, and that it's very easy to take advantage of. Because while it walls half of the metagame, there are also quite a few Pokemon that can put it in a really bad position. Pokemon, such as Mega-Heracross, Mega-Medicham, Mega-Charizard-Y, Landorus-I, basically most wallbreakers can scare it away, and it doesn't help that tease Pokemon also threatens whatever switches in with a lot of damage. Some of these are also known to carry Substitute which makes them even harder to switch out of. Sure, Ferrothorn can Twave them on the switch. But what if it doesn't? You can't perfectly predict what will switch in and when, so it's very easy to find yourself in a nasty situation against these wallbreakers. And yes, it can Protect on MMedi's HJK. But MMedi can also Sub on the Protect. It all boils down to prediction, but if you make one wrong play you put the rest of your team in danger.
Ferrothorn is pretty much the definition of a double edged sword, and this especially shows up in teambuilding. Whenever I decide put Ferrothorn on a team, I suddenly need something that can switch into Landorus, Heatran, MMedi, MHera, etc... otherwise, I'm going to risk losing something every time I have to sand Ferro in. This is all very hard to fit one team, to the point where I become reluctant to use Ferrothorn no matter how many threats it walls. And it really doesn't help that a lot of Pokemon it would be able to beat (Greninja, Latis, Jirachi,) tend to carry coverage to beat it. Oh, and the ubiquity of Magnezone really makes life hard for Ferrothorn too. And unlike Skarm, it needs its leftovers too much to really be able to afford Shed Shell.
There's also the fact that the A+ rank is really, really strong right now. I'm not kidding when I say that every A+ rank besides Talonflame, MDos, YZard and maybe MHera I could conceivably see in S, and Ferrothorn isn't quite at that level.
I honestly think Ferrothron is on the same level as Slowbro. Both wall a large portion of the metagame, are very easy to fit on teams, and are very hard to wear down, but both can give free turns to very dangerous stuff. They have TWave which lets them play around this somewhat, but can still find themselves being a liability for their team if they find themselves facing these threats, no matter how useful they are in the long run. They're very comparable in many ways. So yeah, I feel Ferrothorn should be moved to A, at least until the metagame stops being so hostile to it.