Mega Gallade (Psychic) is basically the new Medicham w/o the HJK misses. Not only that, it has a nice speed tier, boosting moves, and Close Combat. Gallade can also run a stall set w/ Will-o-Wisp + Wish because of its better defenses. Neutrality to Dark moves + Knock Off is nice as well.
Mega Gallade (Fighting) will demolish every mirror match in existence. 110 speed means that it'll outspeed Keldeo, Terrakion etc, making Scarves the only way to hit it. Knock Off will screw potential Doublade switch ins. You lose the 2HKO on Skarm, but it's worth it. Shadow Sneak hits Ghosts hard at +2, and:
+2 252 Atk Gallade Knock Off (97.5 BP) vs. 252 HP / 252+ Def Slowbro: 340-402 (86.2 - 102%) -- 87.5% chance to OHKO after Stealth Rock
Mega Slowbro (Psychic) was put in S rank because it has Heal Bell support making Calm Mind sweeps a lot easier. Water has Heal Bell support, but it's much harder to use. (Assuming Lanturn). Yes, you take up a mega slot, and lose Regenerator but it's worth it once you get rid of its "counters."
Mega Metagross (Steel) is S rank worthy because of its godly stats, typing, and a nice 110 speed. With that, it can outspeed stuff like Charizard-Y, Keldeo, Terrakion etc who'd threaten the team otherwise. Tough Claws boosts contact moves such as Meteor Smash, Bullet Punch, Zen Headbutt etc. Yes, it's still weak to common Fire / Ground / Dark moves, but Heatran and Skarmory can provide the support it needs.
Mega Metagross (Psychic) will face stiff competition w/ Medicham and Gallade, but it has better defenses and a better typing. The only reason why it's in S Rank is because of its ability to preform well in a majority of matchups. I'm a little doubtful on S rank tho.
Mega Altaria (Dragon) is cool, but is simply outclassed by Salamence. Yes, it has a nice immunity to Dragon, but what else? Mega Salamence can run the same set w/ better results because of its higher attack and speed. (110/110/80 attacking stats are pretty disappointing :( )
Mega Latias (Dragon) will be scary to face because of its scary defensive stats. 80/120/150. It also has access to Roost and Calm Mind, letting it be a set up sweeper, or it can be a Defogger. Outclassed by Salamence, but once it's gone..
Mega Latios (Dragon) is outclassed by Latias. Specs hits harder anyways :( The only reason why someone would run Mega Latios is to run a Mixed Dragon Dance set, but that's outclassed by Mega Mence.
Mega Latios (Psychic) is outclassed by all of the other Psychic megas :(
Mega Latias (Psychic) is outclassed as a Defogger (Mew), and Slowbro makes a better Calm Minder because it isn't weak to common types such as Bug and Ice.
I feel like these needed to be explained, and yeah, I'll add them now. Thanks
DM35 <3