Here is my take on the A and A- ranks.
Mega Altaria to A+
Mega Charizard Y stays in A
Mega Diancie stays in A
Dragonite to A-
Excadrill stays in A
Ferrothorn stays in A
Garchomp stays in A
Gliscor to A-
Mega Gyarados to A+
Mega Heracross to A-
Mega Latias stays in A
Mega Lopunny to A+
Mamoswine to A-
Mega Manectric to A-
Mega Medicham to C
Mew to A-
Rotom-W stays in A
Slowbro stays in A
Mega Slowbro to A+
Terrakion to A-
Mega Venusaur to A-
Mega Altaria in A+ rank shouldn't come as a surprise, it's a fantastic Pokemon. Offensively, Dragon Dance sets (either offensive, or bulky) setup on a plethora of Pokemon and are very difficult to revenge kill thanks to Mega Altaria's typing and balanced stat spread which grants it great bulk. Fairy-STAB is probably the best STAB type, Mega Altaria has Roost for keeping itself healthy, special sets lure physical checks, and defensive sets wall a number of Pokemon including Mega Charizard X, Mega Charizard Y, and Mega Sceptile.
Dragonite should move to A- or even B+ because the ORAS metagame gives it far more checks rendering most of its main sets harder to work with effectively. It's checked by Mega Altaria and Mega Metagross (talking about DD sets btw), and CB Dragonite is less useful because with Mega Altaria, Mega Diancie, Sylveon, Clefable, and Mega Metagross all being very common, it has to play a lot more carefully, which isn't desired. CB Extremespeed is still a valuable tool though to check offensive teams that have been weakened a bit, with common Pokemon like Thundurus, Mega Sceptile, Mega Lopunny, Latios, and Greninja, all of which make CB Dragonite still a reasonable choice, so I'm leaning more towards A- rank.
AM explained Gliscor, it loses a lot of its utility with Mega Sableye being close to standard on stall teams, and it's almost funny to think that Mega Sableye almost hinders Pokemon's viability in the way Aegislash did, oddly. Not on the same level, but it's interesting. Gliscor can't touch Mega Sableye; Mega Sableye sets up in its face, not a good plan, and Gliscor isn't that good against offense either due to lacking Intimidate and not being able to use Choice Scarf like Landorus-T, while also being exploitable by the many Pokemon on offensive teams like Greninja, Mega Altaria, Sub Mega Sceptile, SubDD Gyarados, etc.
Mega Gyarados should probably move up because of two reasons, Crunch making it one of the few physical Water-types (or even Pokemon) to be able to get past Slowbro and give it a neutral STAB against many Water-resists like Mega Venusaur, and the fact that Mega Gyarados is able to completely setup against Mega Sableye with Substitute, as Knock Off and Shadow Ball can't break the Substitute. Mega Sableye are often paired with Chesnaught though, but once this is taken care of it's easier. Mega Gyarados is annoyingly walled by Mega Altaria though on offensive teams, so I'm kind of torn.
Mega Lopunny is an exceptional Pokemon in the metagame, Fake Out + base 135 Speed makes Mega Lopunny the new Greninja, lol. Amazing coverage in Scrappy High Jump Kick + Return gives it two free slots to do whatever it wants, and Mega Lopunny has plenty of support options for these slots which have been explained.
Most things have been agreed with by other members of the thread, some of the drops I felt didn't compare to other Pokemon in their ranking, and Mega Medicham is terrible lol there isn't any reason to use people are just looking for niches but it's obvious in a few months time no one will be seeing this Pokemon anywhere.
Not going to touch A- rank just yet, although the pretty obvious outlier is Mega Sableye which is easily A+ at worst, for obviously being a stall staple that checks a number of Pokemon while also being a lategame wincon and immune to Taunt, Toxic, and the rest, yes the stuff we already know.