Let's get this under way!
Mega Altaria (A) -> A+ | This is a no-brainer. Mega Altaria just has so many standout traits that make it perform excellently in OU that there's no reason why it
shouldn't be A+. That Dragon/Fairy-type is just fantastic for both offensive AND defensive capabilities and Cloudburd makes wonderful use of it. Its 110/110/80 offenses initially don't seem much, but Mega Altaria gets Dragon Dance to bolster its Attack and Speed
and it has Pixilate to create some monstrous STABs (Return on the physical, Hyper Voice on the special side), so the stats are good as they are. It can perform defensive roles as well with its 75/110/105 bulk; combined with its typing, Mega Altaria can wall a large variety of Pokémon and keep itself healthy thanks to its access to Roost. It may not have the same power as Mega Charizard X, but it's got more setup opportunities due to the aforementioned traits.
There's just so much Mega Altaria can do and it always pulls its weight, so I'd say
Mega Altaria should soar to A+.
Mega Gyarados (A) -> A+ | There's no denying Mega Gyarados has gotten a ton better in ORAS thanks to its newly-acquired STAB Crunch. Water/Dark is excellent coverage as it is, only resisted by a few Pokémon in OU and even
those have trouble taking boosted hits from this monster. Mega Gyarados has a great base 155 Attack stat and the Mold Breaker Ability, meaning it can hit hard without being stopped in its tracks. Its 81 Speed might not seem like much, but it's one point above Mega Altaria, so it's not all
that bad, honestly. This serpent has great 95/109/130 bulk, which when combined with Intimidate gives it a lot of initial set-up bulk. Intimidate pre-Mega allows this Pokémon to force switches and create many free opportunities to set up at least one Dragon Dance. Mega Gyarados has a very customizable fourth moveslot due to the coverage Water/Dark has; Ice Fang for Mega Altaria, Earthquake for Rotom-W and Taunt to shut Stall down.
Its typing does give it five rather annoying weaknesses, limiting its switch-in opportunities once it has already Mega Evolved, but it does have that huge bulk to back it up. In short, Mega Gyarados is definitely a big threat in ORAS OU and
should go up to A+.
Mega Manectric (A) -> A | I'd really like Mega Manectric to stay where it is. It's one of the best scouts in OU and still does its job as a cleaner excellently. Intimidate, a fast Volt Switch, good Special Attack and Fire-type coverage are all Mega Manectric needs to stand out among the other Electric-types and the other Mega Evolutions in OU. These are all traits unique to Manectric in the tier among its type (yes, I know Zapdos gets Heat Wave, but not Overheat or Flamethrower). It's a fantastic partner to Landorus-T to form a dual-Intimidate VoltTurn core that forces many switches and is a thorn in the side to every physical attacker in the tier. Before you tell me Mega Manectric lacks power; its Thunderbolt usually does just a bit less than Mega Lopunny's Return and considering
that is considered powerful, then either both are weak or both are strong. You decide.
Mega Manectric does have some trouble with Dragon-types and is prone to being set-up bait for Mega Altaria and Mega Charizard X, but its ability to cripple and/or stop some top threats like Mega Metagross, Mega Pinsir and Talonflame just proves Mega Manectric still has a definite spot in OU, so
it should remain in A.
Mega Sceptile (A-) -> A | Why isn't this guy ranked higher? Mega Sceptile has a ton of potential in OU; between its great 110/145 offensive stats (mainly that Special Attack) and fantastic 145 Speed, it's one of the fastest hard hitters in the tier. Its great mixed offensive stats make it so that players can perfectly adapt their Mega Sceptile to whatever their team needs, be it a mixed or special attacker with quite a few coverage options. Mega Sceptile has a nice diversity in Grass-type STABs, too: Giga Drain for longevity, Energy Ball as a reliable spamming move that can force switches with the SDef drop and Leaf Storm as a hit-and-run nuke. Lightning Rod is an excellent Ability that helps Mega Sceptile absorb Thunder Waves from Thundurus-I and stop Volt Switches from Rotom-W (whom Mega Sceptile hardcounters, for that matter), so it's the go-to paralysis absorber on faster teams.
Unfortunately, Grass/Dragon is a shitty defensive typing, giving it six nasty weaknesses, and it's kind of frail with its 70/75/85 defenses, meaning it'll have trouble switching in most of the time. Despite these flaws, Mega Sceptile is an excellent offensive Pokémon in OU and
easily deserves to be in A.
Mega Sableye (A-) -> A+ | Mega Sableye singlehandedly buffed Stall into a much more viable playstyle. It's the very
face of Stall; between its 50/125/115 defenses, Dark/Ghost-type,
gigantic support capability granted by its movepool and plethora of ways to annoy its opposition, Mega Sableye stands as one of the best Pokémon on defensive teams. Magic Bounce perfectly complements its ability to stall and annoy, reflecting status moves and Taunts. That's probably one of the most standout traits of Mega Sableye's: its Taunt immunity as a supporting/stalling Pokémon. It can spread burns like the fucking plague, Taunt just about everything that tries to stand in its way and set up Calm Minds on many Pokémon in the tier, then take whole teams apart with Shadow Ball. Its regular Ability, Prankster, lets it get off a quick Will-O-Wisp or Calm Mind in advance on the turn it Mega Evolves, already giving it a leg up on turn 1.
It has some shortcomings, however. Mega Sableye can't take repeated onslaughts terribly well because of its low 50 HP and it's
slow as molasses with only 20 Speed. The gem gremlin also can't stop Mega Gardevoir from completely taking Stall teams apart. This aside, though, Mega Sableye is an absolute force in the OU tier and the Pokémon responsible for keeping an
entire playstyle stable.
Definitely A+.
More to come later. Kyuzeth out.