Although Mega Gallade is an amazing Pokemon, i think people should wait a bit more before nominating it for S rank, because it's on the same level as Mega Metagross and Mega Salamence right now, the two best MEvos in OU. Stall is very common and it deals with Mega Gallade pretty damn good, be it with Mega Sableye or Thunder Wave Mega Slowbro, musts for stall teams anyway. As for offense, Mega Gallade is unfortunately outsped by many Pokemon found there, such as Greninja, Mega Lopunny, Mega Sceptile, Mega Salamence, Mega Manectric, Mega Aerodactyl, Greninja, Weavile, every single Scarf user, and is also threatened by priority users such as LO Bisharp and Talonflame. In addition, it's not impossible to check for offensive and balanced teams, with Pokemon such as Thunder Wave Clefable, bulky Mega Altaria, Landorus-T, Mega Salamence, Thunder Wave Togekiss, and even physically defensive Klefki being ok to solid checks, in top of the faster Pokemon and priority users that threaten Mega Gallade. Furthermore, it's bulk is just ok and it's typing almost completely irrelevant defensively, meaning it contributes little in terms of defensive synergy, unlike other offensive MEvos such as Sceptile, Aerodactyl, Manectric, and Salamence. Finally, Mega Salamence is a huge pain in the ass and a great check to Mega Gallade, which in top with the other problems is enough to keep Mega Gallade in A+ for now.
Here are the changes i think should happen:
Keldeo: S ---> A+
Latios: S ---> A+
Metagross (Mega): A+ ---> S
Pinsir (Mega): A+ ---> B-
Thundurus: A+ ---> S
Azumarill: A+ ---> A
Garchomp: A+ ---> A
Charizard (Mega Y): A+ ---> A
Venusaur (Mega): A+ ---> A
Keldeo's Speed is not that good anymore, and gets outsped by a large majority of Pokemon found on offensive teams. Furthermore, it got new hard check and counters, such as Mega Salamence, Mega Altaria, Mega Sceptile, Mega Slowbro, and Mega Latias, all of which make it significantly harder for Keldeo to make its presence felt. Keldeo might even deserve to drop to A in the future, but A+ should do for now.
Latios suffers a drop in viability for the same reasons as Keldeo pretty much, with its Speed not being so notable anymore, and gaining some new checks / counters, such as Mega Metagross, Mega Diancie, and Mega Altaria, as well as receiving a bit more competition for the anti-hazard role from both Mega Diancie and Mega Sableye.
Mega Metagross checks a ton of stuff, has amazing bulk for a Pokemon with so much Speed and power, hits hard as fuck, and has a ton of versatility, allowing it to control the pace of the game effortlessly. Be it Agility + 3 attacks, mixed with Grass Knot, or other more situational but very useful moves such as Pursuit, Hidden Power Fire, Stealth Rock, and Bullet Punch, Mega Metagross always does its job on a team. It is by far one of the most influential Pokemon in OU atm.
Mega Pinsir is heavily outclassed by Mega Salamence, with the only thing it has over it being Quick Attack (SD doesn't matter when you can go mixed to smash through physical walls anyway), so B- seems fair, and maybe it should go even lower, this thing is pretty useless with Mega Mence around, though i may be underestimating the merits of Quick Attack a bit.
Thundurus just feels as an S rank threat to me right now. I am seeing hyper offensive and stall teams everywhere, and Thundurus is amazing vs both. Against offense, Thundurus is lucky enough to outspeed base 110s, including new MEvos such as Diancie and Metagross, and provides an invaluable safety net with its priority Thunder Wave that no other Pokemon can do. What makes Prankster Thunder Wave so important now are threats such as Mega Salamence, Mega Lopunny, Greninja, Agility Mega Metagross, DD Mega Altaria, and more, all of which threaten to destroy offensive teams in a heartbeat. As for stall teams, a simple NP + 3 attacks or mixed set with Knock Off can soften everything up or even sweep on its own, while still having that sweet 111 Speed to outrun many important offensive Pokemon. If there is a reason why Thundurus doesn't deserve to be in S rank that would be its inability to significantly threaten both offensive and defensive teams simultaneously, because it can't break through stall effectively without NP + 3 attacks or 4 attacks mixed sets, and it can't have Thunder Wave for offense if it chooses to break past stall. Not 100% sure about Thundurus for S rank, but i am leaning towards S, as priority Thunder Wave has once again become the amazing revenge killing tool it used to be.
Azumarill should drop because it received new checks in Mega Metagross and Mega Slowbro, while some of the Pokemon it used to check are either not that important or it can't check them anymore, such as Greninja and Keldeo. And it can't even check properly the new Megas that it's typing suggests, such as Mega Diancie, Mega Lopunny, and Mega Altaria, because they can all 2HKO it by targeting its mediocre defensive side.
The only reason Garchomp was worthy of A+ rank for me was because of its Stealth Rock set, a very consistent SR setter with sufficient offensive pressure to threaten any kind of anti-hazard Pokemon. With the arrival of Mega Sableye, this is no longer the case, as Mega Sableye hard walls non-SD Garchomp sets and acts as a good check to SD sets. This means that offensive teams will have to rely on other, more effective SR setters, such as Mega Diancie, Clefable, and Heatran, and Garchomp should drop to A to reflect this.
Mega Charizard Y received two major checks in Mega Altaria and Mega Salamence, while it's placement in A+ was questionable even before ORAS, as it needed too much support in comparison with the other Pokemon in A+ rank, so dropping it in A makes sense.
Mega Venusaur was an amazing Pokemon in XY thanks to its ability to handle some of the best Pokemon in the metagame, including Azumarill and Keldeo. However, those Pokemon are not nearly as effective in ORAS as they were in XY, making Mega Venusaur's qualities less needed. Add some very threatening new Pokemon that can turn Mega Venusaur into a liability, such as Mega Metagross, Mega Latias, and Mega Salamence, and it's easy to see why this drop needs to happen.
Finally, some people discussed about Mega Charizard X and Mega Gardevoir dropping. I am not sure and i don't feel strong about this, so i am taking a neutral stance about this for now, though it's worth noting that Mega Charizard X is still very effective against offensive teams and that Mega Gardevor still does a very good job at dealing with stall. Also, bulky SD Mega Charizard X is an amazing check to Pokemon such as Mega Slowbro and Sableye, which must count for something, considering they are some of the best bulky sweepers in OU and are considered very hard to deal with.
EDIT: Oh and definitely Mew for A or even A-, it can't stallbreak shit with Mega Sableye around.