this list is a little weird to be honest.
I agree with what dadoux
said; and here's what I think should be changed on that list.
Garchomp to S: agree with sogeking.
Gliscor to A- from B+: I'd possibly place it higher but Gliscor sitting in the same rank as Lucario and Tornadus and just one rank above Feraligatr and Zapdos is laughable. Gliscor's SubToxic set is one of the most dangerous sets in the metagame and Landorus can't run that. Landorus is probably a better pivot on offensive teams but that's not the only thing Gliscor can do. On top of that, the double dancer set is pretty neat and Hyper Cutter + Flying STAB can wreck some of its common answers, like Gyarados or more importantly Landorus-T.
Stoutland to A- from B+: Stoutland is a pretty neat sand abuser and it can work on both offensive and semi-stall teams which increases its diversity, even though it can only really run 1 set. It's one of the few mons which makes you run Hippowdon or Tyranitar for their weather and not for fighting off other weathers / them being good mons.
Mew to A- from B+: The stallbreaker set is incredible especially since stall-ish teams are pretty popular now. So many teams can't do shit to Mew, and it can pull its weight in most games. This thing can run anything it wants really but I think Taunt+WoW / NP / Lead are all viable enough to justify A-. Typing's not too good but a Fighting resist is always appreciated in a tier full of those.
Gastrodon to A- from B: Might be a long shot but I find slug to be pretty fucking good in the current meta. It shuts down most Rain offenses very easily and basically means you can afford to run only 1 Water resist which is nearly unheard of in BW. One could say Gastro's p easy to take advantage of but the LO set punishes switch ins hard and has nice coverage, while still maintaining great utility. If A- is too high, then rise it to B+ for sure.
Salamence to A- from A: Salamence is kinda garbage. It's not even outclassed by Dragonite, it's just... meh. The Scarf set is good on hyper offense but that's like the only thing it does well. The mixed set works.. sometimes. Salamence has nice resists but if I wanted to use it for the resists I'd just run Dragonite. IDK
Gengar to A- from A: Another PkMn that's I find to be p average. It can't really block any spinner which sucks, especially when Jellicent does it p well for balance and stall. It has a niche on Custap Skarm teams but that's about it. Not worthy of A rank IMO
Volcarona to A+ from A: BROKEN AS FUCK LOL
Also, I think too many mons are in A+. A+ should really be for the best mons in the meta and it just seems to be a collection of good mons atm... Haven't really thought this through, but I think Alakazam / Celebi / Hippowdon / Rotom-W could all go down to A.