Just posting to address some largely neglected comments and suggestions, since a lot of them deserve to be addressed properly.
In other news, Ditto still needs to be ranked as I mentioned a couple of pages ago. Around C-Rank will do, as it is definitely a viable option and has the niche of flat out copying the opponent plus its stat changes, potentially causing switch-outs, revenge kills, or in some cases, even sweeping up of teams. However, its effectiveness in battle is ultimately situational and because Ditto almost always runs a Choice Scarf, it can be easily worked around, and in some cases, it can even become a huge liability as choosing the wrong move can result in giving opponents free turns to wreak potential havoc on your team.
Ditto needs a rank and C is appropriate.
Ditto is interesting. It has substantial advantages in its potential versatility, the ability to use most anything an opponent can against them and above all the ability to countersweep an opponent with their set up sweepers, which is often enough to prevent them from setting up at all. On the other hand, it tends to lack longevity and utility outside revenge killing, is very limited against stall and therefore somewhat match up reliant, it lacks the ability to check Calm Mind Arceus and can be played around. All in all it is relatively niche.
Mewtwo: S --------> A+ Mewtwo is a powerful threat, don't get me wrong, but it's power doesn't seem enough to merit a place in S rank. Once it loses it's unpredictability by mega evolving it's just another Pokemon. It's typing is rather shitty, its bulk is rather mediocre and it has trouble OHKOing just about everything unless its MMX and is actually hitting for SE damage. The different formes can fit multiple roles but they just don't seem to be defining enough to keep it in S rank. The presence of Yveltal and rise of Ghostceus means that the metagame is beginning to work against it, limiting it even more. Its strapped for moveslots to hit everything hard while also lacking the moveslots to either set up, or recover most of the time. Mewtwo is a powerful threat but in my opinion its just not THAT good for S rank.
Although I can see that Mewtwo sometimes seems to underperform, as its coverage is somewhat match up reliant and certain Pokemon will check the chosen variant very well, it definitely deserves its place in the S rank.
To begin with Mega Mewtwo Y, which I consider to be the superior Mega Evolution, its stat combination and coverage options are incredible. The speed is so great that very little can outspeed it unboosted, and almost everything that can outspeed it (unboosted or otherwise) struggles to switch in for fear of being OHKOed or 2HKOed by its STAB (and revenge killers can rarely afford to sustain such damage). The power and coverage it has threatens to OHKO or 2HKO all but the most robust walls, although of course it can only use a few coverage options at a time, leaving it walled by certain Pokemon. Nonetheless, very unfavourable match ups aside, the combination of this power, coverage and speed, complemented by Taunt allows it to threaten everything from offensive to full stall teams quite effectively. Finally, the increased special bulk is surprisingly useful, even at the cost of physical bulk; the offensive teams on which Mewtwo is often found commonly lack good tanks for attacks beyond their Arceus forme, which can easily be overwhelmed if it must take on this role alone. Many such weaken the offensive momentum on which these teams thrive. Mega Mewtwo Y's good special bulk is often just enough to ease this role a little for Arceus; it can even survive any hit from a slightly weakened Scarf Kyogre, and its superb base speed allows it to forgo some speed investment to supplement its bulk. Also, do not underestimate the unpredictability of it once revealed; between various coverage options and Calm Mind vs Taunt variants, its checks vary substantially. Admittedly, its typing is underwhelming, it is Pursuit-weak if it lacks coverage to threaten the Pursuit user (although Fire Blast/Focus Blast are very common on it) and it is very vulnerable to physical attacks, but it is nonetheless a major threat.
Mega Mewtwo X I am admittedly less familiar with, but it is still a potent threat. It has a slightly less elite speed tier in exchange for a more devastating STAB, and better mixed but worse specialized bulk. Almost every point about Mega Mewtwo Y's speed, access to Taunt and power applies for Mega Mewtwo X. It also has some good coverage options and the ability to use effective mixed sets (which Mega Mewtwo Y does not need to owing the Psystrike mechanics), allowing it to overcome the vast majority of its checks like Mega Mewtwo Y, although a lack of good Dark/Ghost coverage leaves it struggling with Ghosts, and many of these coverage options are not as powerful as those Mega Mewtwo Y has. Nonetheless, the power of its Low Kick cannot be understated, as it comes close to OHKOing even neutral targets such as Palkia and Kyogre. Its typing is again underwhelming, where an appreciable loss of the Dark-type weakness arguably does not compensate the gained Fairy-type weakness, while Bug and Flying are both relatively rare attacking types. The loss of Pursuit weakness and lesser frailty against physical attacks are useful, although Mega Mewtwo Y's specialized bulk arguably has more overall utility.
Finally, there is Stalltwo, which should certainly not be forgotten. Although not threatening in terms of KO potential, it can still be debilitating to face for many more defensively oriented teams, and hamper them even more than Mega Mewtwo X or Mega Mewtwo Y as they rarely carry Recover. Its greater bulk and use of Recover and Will-O-Wisp also allows it to check various threats repeatedly, which more offensive Mewtwo variants cannot do.
I also feel that the offensive variants are not checked as effectively by Yveltal as Ghost Arceus is, as Yveltal cannot switch in nearly as easily even against common moves (whereas Ghost Arceus must use niche coverage to threaten it), often being 2HKOed or even OHKOed after SR by the right move. It can revenge kill both sets very well and excels against Stalltwo, but it does not happily switch into Mega Mewtwo Y and it can rarely switch into Mega Mewtwo X at all. Mega Mewtwo Y can also overcome Ghost Arceus; some EV spreads struggle to switch into Psystrike, none of the common ones can survive two Shadow Balls, and Calm Mind variants simultaneously threaten Ghost Arceus more and are less vulnerable to Judgment. The other two Mewtwo still lose to Ghost Arceus though.
With respect to that I think Scizor needs to drop to at least A- Rank if not below. With respect to meta adaption, the current common coverage on the 2 pokemon it is used primarily to check - ekiller and xerneas - both of them can beat it with a relatively common coverage move. Its other utility doesn't otherwise allow it to shine. Don't think I have to say too much, but it is definitely a lot worse than we thought it was in early days.
Scizor for A- rank.
With little regard for the recent post that is likely the work of Jimmyftw, I too feel that Scizor is ranked a little too highly. The metagame quickly adapted around its hype and Fire type-moves are relatively common on both Geomancy Xerneas and Extremekiller Arceus, leaving it unable to do more than revenge kill the former and completely unable to check certain variants of the latter. It can also serve in other roles, such as soft checking Calm Mind Arceus with Toxic, acting as a functional (but not effective) Defog user, or Pursuit-trapping Gengar for teams that would otherwise be hampered by it, but it can only perform a few of these roles at a time. Finally, it is easily exploited by many major metagame threats (unless it carries U-turn and can predict these switches, but this compounds its 4MSS). It has a lot of varied utility to provide teams, but the metagame developed against it somewhat and again it can only perform some of its roles at a time.
IMO Deoxys-S and Lugia are way too high. Deoxys-S literally lost every niche thanks to Defog. It just can't run an offensive set and it's oretty easy to Defog on, and sure you can Taunt a support Arceus or something but you can't do anything back so you just end up losing anyway. Lugia is bad, don't think I need to explain much about that, since everything is running Toxic etc
Deoxys-S is still the face of hyper offense. The small advantage in speed and big advantage in bulk just makes it a lot more troublesome than Deoxys-A could hope to be as a double entry hazard setter, not to mention that using Taunt, Stealth Rock and Spikes on Deoxys-A effectively negates its advantage over Deoxys-S since the offensive capability between a Focus Sash and no coverage is lacking (while lacking any of those moves means they are no longer performing the same role). Even when antilead, it can sometimes come in a few turns later and manage to set 2 layers of hazards. Once it has successfully set hazards and died, a well-constructed hyper offensive team should have little difficulty either preventing or significantly delaying the removal of entry hazards with a combination of Taunt and the offensive threat of giving free turns to devastating set up sweepers, as well as potentially Darkrai's Dark Void or a Rock Polish + Explosion Landorus-T. With entry hazards maintained for a handful of turns and abused freely by major offensive threats, the opposing team will likely be too broken to manage even if the hazards are removed.
Not addressing Lugia, Hack can do that far better than I can.