Hi o.o thought I'd post my impressions on A/ A- ranks as I haven't done anything in this thread. I'll try to keep each explanation concise. Sorry if I'm repeating anyone, as I haven't been actively reading thread. Whatever I don't mention I believe deserves its current rank/ will very soon.
Things I'm unsure of/ haven't had enough experience as of yet (cannot decide whether to move up or down)
Mamoswine
Aerodactyl
Diancie (believe it should be A+ as of now but wary of how meta will treat it)
Manectric
Altaria (Mega) A ->A+: Altaria can perform a ridiculous amount of roles in a dragon dance sweeper, special attacker, cleric etc. It has the ability to lure in its counters (Heatran and Skarmory) of commonly used sets and kill them with earthquake and fire blast respectively. Whether it will be as effective down the line as it is now is unlikely, but as of now Altaria is a massive threat for a variety of teams and playstyles depending on which set you choose, all of which are viable.
Dragonite A -> A-: Dragonite's two main sets, dragon dance and choice band, have both fallen a bit in viability. It doesn't have many opportunities to set up with Dragon dance in such an offensively paced meta, and its cb set also suffered, although banded extremespeed is still very useful and is still threatening.
Gyarados (Mega)
A -> A+: Gyara's insane bulk, ability, typing and addition to a well-needed stab move in crunch have all benefited it in this meta. Crunch is extremely useful in killing Slowbro, one of its premier counters in xy. It's sub/DD set has the ability to completely wall and set up on Sableye, a massive pain in ORAS as of now, and can still check extremely annoying threats like Bisharp.
Gliscor A -> A-/ B+: Gliscor, although still not a bad pokemon, faces much more offensive pressure this time around, being unable to check many new physical threats in Gallade, Metagross, and Altaria, as well as having much more difficulty stallbreaking with Mega-Sableye.
Slowbro (Mega) A -> A+: Megabro initially received insane buzz, then died down quickly after it was released. From what I've seen, people have been using it incorrectly, assuming it can take on special threats without taking any boosts, crippling it for future use. Not only does its non-mevo get free health on switches, it's mevo's ability prevents critical hits, allowing you to stack up stat boosters like acid armor and calm mind, making it almost impossible to kill without status. In addition, it can run an offensive cm set, as well as a set similar to crocune. Whatever you choose, megabro is an insane threat and extremely difficult to deal with when used correctly.
Heracross (Mega) A -> A-: What I thought was actually overrated in XY, mega heracross has gotten much less viable in ORAS. It is obviously extremely strong and has great coverage, but it's simply too slow to deal with most teams, although it will still be an indefinite pain for stall (which is also extremely overrated in ORAS)
Mew A -> A-: Not quite as useless as it appears to be, mew has definitely decreased in viability in its stallbreaker set due to Mega-Sableye. However, Mew is fast, bulky and has a unique typing along with an extremely large movepool, and would be downright wrong to remove from the A ranks.
Venusaur (Mega) A -> A-: Venusaur has taken a huge hit in viability, making it difficult to justify a megastone with Mega-Sableye, Mega-Slowbro and Mega-Altaria common choices for stall, along with Metagross and Gallade breaking through it easily. It is by no means a bad pokemon, but is not as easy to add to your team as it was before in xy.
Medicham (Mega) A -> C+: Medicham was a complete monster in xy, but is almost completely outclassed by the faster Gallade that has access to swords dance and better coverage. I can still see it functioning as a setup-free wallbreaker, considering its HJK is still insane, and has access to two forms of priority.
Manaphy A- -> A: With venu being less of a factor, Manaphy has even more opportunity to shine, having the ability to singlehandedly annihilate stall. It can run a boosting set with scald / coverage move along with tail glow and rain dance, or can also run rest to ensure reliable recovery. Either way, the current stall archetype gets completely destroyed by manaphy, and should not be overlooked in teambuilding.
Sableye (Mega) A- -> A+: I'm sure this has been explained already. It's not ready for S-rank, however.
Sceptile (Mega) A- -> B+: Sceptile is fast, and provides a unique dual stab. Unfortunately, it isn't particularly strong, is very easy to switch into for stall, has weak stab moves and essentially is required to both rely on focus blast and leaf storm (lowers SpA by two stages), as giga drain is simply too weak. It has potential to move back because of how it can deal with offense, but from what I've seen it simply isn't good enough.
I think a lot of people believe scept should stay in A-, but I still don't. It can be easily compared to Manectric (potentially) in the same viability rank. Both scept and mane are fast and strong, with scept being faster and nominally stronger.
However, Manectric has a more useful ability in intimidate, is also immune to t-wave, better coverage, and the ability to gain momentum against stall in volt switch. In addition, Manectric can check a lot of threatening pokemon like talonflame, thundurus and scizor, while scept has extreme trouble switching in on anything.
The way I see it, everything else is where it deserves. Thanks for reading! If you want me to justify why something should stay where it is, just let me know.