Ferrothorn Placement: B-Rank, at best. While that 4x Fire weakness does cripple it, it does have access to various support moves, such as T-Wave, Leech Seed, and Stealth Rock. I disagree (somewhat) with both Sound and nyttyn that Swords Dance is the only really viable set that Ferrothorn can use, As it can wreck many pokes with just Gyro Ball, no boosts, combined with something like either a Sassy or Brave Nature, but I do see the necessity for SD to take out some of the more bulky pokes, like Garchomp or Kangaskhan. But, with the rising of the Sun teams (see what I did there) from the addition of Mega Charizard-Y, Ferrothorn has become much harder to use without worrying about getting Fire Blast-ed, so I think it would be hard for it to be an A-Rank pokemon, while it does have a lot of positive traits which keeps out of C-Rank. So, I think B-Rank would be the best for Ferrothorn.
Gourgeist B or C Rank: Not sure what to say about Gourgeist. Trevenant has a lot of things, mainly Harvest, that make it a better pokemon than Gourgeist, but Gourgeist has access to Shadow Sneak and can have a variation in its stats depending on its size. The Super Sized form probably has the best stats because it's the most bulky of the four sizes. Trick-or-Treat could find some sort niche in Doubles, though I doubt that I will see that happen do to the complexities it could bring. Put it in C-Rank.
Conkeldurr rising to A: What's not to love about Conkeldurr? It has good bulk, an excellent attack stat, is probably the best Guts user, and has a movepool that can complement it quite nicely. However, it's slow as heck and can get demolished by Psychic-types like Gardevoir and Alakazam with it's low Sp.Def.. Nevertheless, it can also work with Trick Room teams to make up for that low speed, and by having access to Wide Guard, as well as having Drain Punch, allows it to use a variety of different sets to meet whatever is needed and is pretty cool. I would say that Conkeldurr has got enough to make it to A-Rank.
Herracross being added to B: I vote for yes. Heracross now has a lot more sets, or more utility, to use than seen in previous gens with the addition of his Mega Evolution. They gave Heracross Arm Thrust, Bullet Seed, and Rock Blast this gen just so that we all may abuse Skill Link on Mega Heracross. Mega Heracross is much like Cloyster, except that he doesn't really need to set up in order to do a lot of damage and has a greater bulk (80/115/105) that's much more rounded than Cloyster. Moxie would be the best ability for Heracross if your going to mega evolve it, as it gives you an attack boost for whenever you KO an opposing pokemon. Sure, that 4x Flying weakness just murders this pokemon, but with some good teambuilding and making smart choices, Heracross can be a scary threat to deal with. I learned that the hard way on Showdown. So yes, Heracross should be added to B-Rank.
Whimsicott rising to B: I support the the bump up for Whimsicott. Having priority Tailwind can prove to be very important to the outcome of any battle, especially when you're up against a foe who has a lot of fast pokemon, like Greninja or Gengar. Prankster is what makes this pokemon great, but other than being a speedy supporter, it really can't do much else. With now being part Fairy, it can get easily crippled by Poison types, and is also now weak to Steel, which is still a fairly dominant type. But, having priority for all its status moves can prove key to any victory; it's not just Tailwind that's important. So give it the placement boost it deserves, put Whimsicott in B-Rank.
Parasect being added to C: Amoonguss is not the only TR Spore and Rage Powder user that's available, as Parasect is able to perform that role with almost the same efficiency. However, it has two 4x weaknesses, Fire and Flying, that can cripple it, and having Dry Skin does not help with reducing the damage of any sort of Fire-type moves. Effect Spore is kind of sketchy, and who even uses Damp? The moveset is probably the only promising thing that Parasect has, with moves such as Wide Guard, Leech Seed, Aromatherapy, and obviously both Rage Powder and Spore. C-Rank is the best option for Parasect, as its advantages clearly do not mitigate its disadvantages, but it is able to work in Doubles.
Mr. Mime being added to C: If Mr. Mine was being added to B-Rank or higher, I would of said no. But C-Rank is a safe choice for Mr. Mime, given that it needs a lot of team support if it wants to perform well, and those are part of the requirements for C-Rank.
Jellicent rising (B or A?): Too Bulky. That's what I can say about Jellicent. While its Def. is mediocre (100/70/105), when Jellicent has both Acid Armor and Will-O-Wisp, it can get hard to KO it. Jellicent has access to a lot of recovery options: Pain Split, Giga Drain (Transfer-Only), and obviously Recover. It ability Water Absorb also helps in maintaining its almost unstoppable bulk, and Cursed Body helps make sure no one tries getting physical with it. Finally, as Nollan noted out, Jellicent has 6 resistances, which helping in its walling capabilities So put Jellicent in A-Rank because this pokemon is just too bulky to handle.
Virizion being added to B: Not too sure. I rarely used Virizion in 5th Gen, and it will probably be the same in 6th for me. All I can say is that Whimsicott can be a nice complement to it, since Whimsicott can use Beat Up and Virizon's ability is Justified. Beat Up(5x)+Justified=Massive amounts of damage and sweeping. I really don't have the calculations for it, but that's the best I can do and was a cool combo I noticed. Nollan makes a good point about what Virizion should be used for, and I agree that it can be used to take out/check common threats that are found in both A-Rank and S-Rank (not Heatran, though). I'm going for Virizion being in B-Rank.
Torterra being added to C: Yes. Wide Guard now gives it some utility, for once. I see what you mean Lokomori about Heatran not being able to switch while Torterra is active, but what if the Heatran is holding an Air Balloon? what will you do then? I'm still worried about that 4x Ice weakness, but otherwise, I'm for Torterra for C-Rank.
Lucario rising to B: Hard to argue about. I still think that giving a mega evolution to Lucario seemed pretty pointless, since it was already pretty strong, but now it's just way too powerful. The only con that seems pretty big is how frail Lucario is, even with his Mega, and so it's pretty easy to KO him, but it can make up for that disadvantage with the brute strength it can bring. Because of this, Lucario does deserve to go to B-Rank.
Rotom-C rising to B: Electric/Grass is an interesting type combination, as it makes Rotom-C immune to being paralyzed as well as all powder moves, and the Electric-typing neutralizes many of the weaknesses that Grass-types face. While I do dislike Leaf Storm somewhat, mainly due to the Sp. Atk. decrease it brings after being used, Rotom-C has Charge Beam to make up for that problem. The big issue I have with Rotom-C is the same issue with Ferrothorn, and that is that Drought teams have seen a great boost in this gen, which has lead to an increased usage of Fire-types. I think it would be best just to keep Rotom-C in C-Rank, as it fails to hop over many hurdles that weren't created by itself, but rather external forces, such as Mega Charizard-Y.