I'd like to make a case to increase a really underrated Pokemon's rank: Ditto. I've been using this guy recently and I strongly believe that the current XY metagame has shaped up in a way such that Ditto becomes significantly more of a threat than in past generations. Firstly, its scouting utility is absolutely banging. It helps you instantly determine unpredictable Pokemon's sets, most notably Genesect, and allows you to act accordingly to that. I mean, the metagame is riddled with Pokemon that can run a plethora of different sets and even run surprise moves to take out lures; examples of these include Thundurus, Azumarill, Charizard, Mega Venusaur, and the previously mentioned Genesect. The scouting utility alone is enough to bring Ditto out of the rank it currently resides in, as it significantly aids in checking and countering your opposition once you now know exactly what you're up against. The other thing about it is it can seriously screw with your opponent when it copies their Pokemon. It's just another play you can make when trying to take on something, such as Genesect's U-turn (the best part about this is you can potentially get +2 Attack or +1 Special Attack/Attack), Mega Venusaur's Giga Drain, Landorus-T's Earthquake, and a myriad of other things. Finally, just the fact that Choice Scarf Ditto checks literally every single boosting sweeper or just basic all-out attackers is unreal. Not only will the opponent actually hesitate to boost in fear of Ditto, it easily prevents you from getting swept by something like Dragon Dance Gyarados and Charizard-X, Tail Glow Manaphy, Swords Dance Diggersby, and Nasty Plot Thundurus if they do choose to boost. This fact can significantly damage the opponent, or even sweep them clean altogether, when you're rolling with an exact copy of their sweeper with 1.5x the Speed. I love that about it. So, with all of that considered, I'd like to take Ditto out of C territory and nominate it for B- rank.
I second this nomination... Ditto's information gathering is such an enormous tool and on top of it, his ability to precisely gather the said information on top of being a deterrent to teams with boosting sweepers makes him all the more mentionable. Here are the definitions for B and C Class Pokémon:
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.
C Rank: Reserved for Pokemon who can be effective given the right support, but either have crippling flaws that prevent them from consistently executing their strategy or are typically inferior to Pokemon in the above ranks.
Thus, based on the definitions provided, Ditto can always activate his Imposter ability and continue to wreck havoc on the opposing team; making him ineligible for C Rank and somewhere in B Rank.