It probably doesn't actually deserve A- rank, but considering how pretty much all that needs to be said about the current As and A-s have been discussed, and I'm starting to read shit like "Trevenant for S-rank", fuck it. Chesnaught needs to move up, this thing is criminally underrated. It manhandles so many A Rank+ heavy hitters like Aegislash, Tyranitar, Garchomp, Excadrill, Azumarill, Gyarados, Landorus, Gengar, Bisharp etc, you name it. It has utility with moves like Toxic, Spikes, and Spiky Shield, it can keep itself healthy with Leech Seed and Synthesis, and it hits surprisingly hard with Hammer Arm and Wood Hammer coming off 107 base Attack. On paper it doesn't seem that great with it's plethora of weaknesses, but after actually using this thing, I personally felt it pulls it's weight more than the comparable Ferrothorn who is currently listed as B+. Yeah it gets vaporized by the Charizards and Pinsir, but B- is way too low for what it brings to the table.
I agree that chesnaught is pretty cool. Being able to counter Aegislash is always great in my eyes, but I think you're over-exaggerating him a little bit. Garchomp likes to carry Fire Blast a lot, and if its mega it will hit that much harder. How are you manhandling azumarill when you're weak to Play Rough? And a Non-mega gyarados with either Sub or Taunt can win against Chesnaught eventually. Landorus like to carry psychic and Sludge Wave, and Gengar can also carry Sludge Wave over Sludge Bomb. Everything else I agree. I can definitely confirm that he carries more weight than ferrothorn does, deserves B+ at the least.
Nominating Bisharp and Mega Tyranitar for A+ Rank
Mega Tyranitar is hands down one of the most threatening MEVos in XY OU and is the best Dragon Dance user, second only to Mega Charizard X. Speaking from extensive usage, Mega Tyranitar is an extremely consistent and reliable win-condition for many games due to its amazing offensive coverage and simply godly defensive stats. A Mega Tyranitar is hard to revenge-kill if the opposition lacks a Scarfed Fighting Type or Technician Boosted Mach Punch. Mega Tyranitar can stomach Banded Bullet Punch from Scizor and also Banded Aqua Jet from Azumarill while eating up Life Orb Greninja's Hydro Pump. These are simply testament to Mega Tyranitar's impressive bulk. Easily being the second most threatening Dragon Dance sweeper in OU due to Mega Tyranitar's sheer power, bulk and set-up opportunities just littered throughout OU, I feel like Mega Tyranitar has what it takes to rank in A+
Can certainly agree that DD mega Tyranitar is a monster. Jeez, I made a warstory on him, back in the age of mega kang. I hopped on the bandwagon for this guy before most people, and for good reason. He sets up on a ton of things, and remember, its taking moves like banded bullet punch from scizor and banded aqua jets from azumarill with
no investment at all. It's ridiculous how bulky this guy is, and with the new buff to speed, at +1 he outspeeds everything relevant that is unboosted. Either move just him up to A+ or move tyranitar up to A+ too, because he's just too good for A.
As for the keldeo argument, we can relate this right back to mega tyranitar too.
Specs keldeo, in particular (don't run scarf guys, its not great) forces in practically everything mega tyranitar sets up on (except mega venusaur, but he won't appreciate sand). There's your perfectly fine offensive core right there.
As said previously, usage doesn't mean anything unless you're talking about SPL usage stats. And if you go see those, keldeo will be skyhigh, I guarantee it.
Keldeo's counters in particular, are very easy to work around. Lati@s are pretty easy to pursuit trap, especially when you have things like Aegislash, AV scizor, AV bisharp, and AV tyrantar about. Mega Venusaur is easy to handle with pokemon like Talonflame and Landorus. This goes back to our wonderful, tried and true, Lando/Ttar/Pony core that was mentioned, which still works fine for the most part. It's pretty much irreplaceable in these cores.
Keldeo for A.