I have a good mewtwo team that is 2-1 in seasonals right now. I won't actually showcase the team because I may use it in future games, but here are replays of the team in action and some points to why I think it should remain in the A subranks.
Vs Bughouse -
http://replay.pokemonshowdown.com/ubers-256766131: This game didn't go all too well for me. Bughouse was running some questionable sets that did not suit his team such as 0 attacks Clefable, Knock Off Sableye, and Ice Beam Waterceus which my team greatly underestimated. Mewtwo's performance was exemplary in this match, as it pressured Bughouse's defogger and magic bouncer very well with Calm Mind alongside a spikes setter.
But it cannot go unnoticed in this replay that Life Orb Mewtwo would have been the superior set in this match. MMY came just short of taking out Lugia from the match (a feat which Life orb Mewtwo would have accomplished) and was unable to break through toxic+protect+wish Clefable because it came just short of power in order to eliminate this threat. Though aside from the point, Lugia must have had some clutch EVs because +1 Ice Beam's lowest role on Lugia is 65%, and after Stealth Rock damage and Leftovers recovery Lugia sat at 63.5%.
- +1 252 SpA Mega Mewtwo Y Ice Beam vs. 252 HP / 4 SpD Lugia: 272-322 (65.3 - 77.4%) -- guaranteed 2HKO after Leftovers recovery
Had bughouse carried a standard Lugia, Mewtwo would have just steamrolled through his team. My team does not opt for Life Orb Mewtwo because Mewtwo Y's immunity to sleep offers such great utility against Darkrai which my team would otherwise have little counterplay against; the higher speed tier also helps keep Mega Gengar in check as the glue of my team, Waterceus, is vulnerable to it. For the reasons stated I do believe Mewtwo Y is more viable than Mewtwo because the marginal increase in speed and actual utility of a sleep immunity makes it useful, easier to fit onto teams, and worthwhile as a choice for a Mega. It should be placed at least a sub rank higher than Mewtwo.
Vs Haruno -
http://replay.pokemonshowdown.com/ubers-261055124: Mewtwo had actually done nothing in this match, but in the matchup it did source a way of revenge killing SD Arceus had something gone wrong in my playmaking that allowed it to set up.
- 252 SpA Mega Mewtwo Y Ice Beam vs. 0 HP / 4 SpD Arceus-Ground: 226-268 (59.3 - 70.3%) -- guaranteed 2HKO
Generally, the ability to revenge kill a boosted non-espeed Arceus is a great quality to have. So if the Groundceus attempts to set up on anything specific to my team (bar Klefki, but in that case Giratina-O is there), Mewtwo will be able to revenge kill.
Vs Croven -
http://replay.pokemonshowdown.com/ubers-262518617: This stresses the point that Mewtwo is an exceptional revenge killer. Croven's team had nothing to take on fast Ice coverage and it successfully pinned down Ho-Oh because Mewtwo would have 2HKOed anything he would switched in. Taunt would have been helpful here to stop the Defog and keep his yveltal dead on a switchin to SR but Ice Beam still took care of the Latias and allowed Giratina-O to possibly pick it off with shadow sneak had it switched out and came in later. Mewtwo did revenge kill Arceus as explained above and below that it is able to.
Comparisons with similar Pokemon -- I won't try to argue that it is better at revenge killing Arceus Ground than something say Skymin is because it isn't. In 1v1 situations against unboosted SD Arceus (especially Ghost) Skymin is always better because of the mere type disadvantage Mewtwo has when facing it—Mewtwo is either forced to switch out on shadow force or KOed by Shadow Claw. If Ghostceus is in fact the rare Shadow Claw + Shadow Force set, Skymin is both better in the scenarios of Arc being boosted and unboosted; in the case of the former it can at least fish for two flinches and then seed flare. Though if your team is weak enough to SD ghostceus, Darkrai should be considered over both Mewtwo and Skymin. But nonetheless, both Mewtwo and Skymin acheive 2HKOs on 0 hp / 0 def Arceus:
- 252 SpA Mega Mewtwo Y Psystrike vs. 0 HP / 0 Def Arceus: 190-225 (49.8 - 59%) -- 99.6% chance to 2HKO
- 252 SpA Life Orb Shaymin-S Seed Flare vs. 0 HP / 0 SpD Arceus: 207-243 (54.3 - 63.7%) -- guaranteed 2HKO
But what holds in the Mewtwo vs Skymin comparisons is that Mewtwo is a more reliable answer to Darkrai than Skymin in a 1v1 because the chances of landing a Focus Blast are greater than the chances of KOing with Seed Flare:
- 70% accuracy >
252 SpA Life Orb Shaymin-S Seed Flare vs. 4 HP / 0 SpD Darkrai: 265-312 (93.9 - 110.6%) -- 62.5% chance to OHKO
Even worse chances of stopping Darkrai with two Air Slash flinches. The only time you actually want Skymin instead of Mewtwo for checking Darkrai is when it is holding a Focus Sash. But how relevant is that set outside of like ladder?
In summation, all three of Darkrai, Skymin, and Mewtwo are used to stop defogs. Darkrai is most definitely the best of the three, hence why it is at such a high rank. Dark Void+Nasty plot is excellent and there is no sizable comparison within this tier that can pressure defoggers as well as it can. Skymin is by far the worst of the three. Its trump card and crutch in Air Slash flinches are successful only 60% of the time and the only relevant defogger it will always beat is Arceus Water because of super effective seed flare. Mewtwo on the other hand is overall very consistent at stopping defogs. Access to taunt is menacing to Lugia and Waterceus, and can leave Latis vulnerable as set up fodder. It is unfortunate that Mewtwo fails to stop Ghostceus's Defog and live to tell the tale, but as steelphoenix has mentioned Defog sets have diminished in usage to nigh nonexistence. Mewtwo also defeats the magic bouncers, however Mega Sableye can be of some trouble because Mewtwo will often have to make predictions in order to eliminate it; though, this is impressive:
- 252 SpA Mega Mewtwo Y Fire Blast vs. 252 HP / 0 SpD Mega Sableye: 145-171 (47.6 - 56.2%) -- 84.4% chance to 2HKO
I don't particularly think Fire Blast is Mewtwo's best coverage option but it does allow Mewtwo to beat Sab and also Klefki, and can't be disregarded. The point here is, Mewtwo can defeat hazard control cores.
CM wars with Arcs!!! Ghostceus is the best one, but Mewtwo beats all the other ones such as Rockceus and the rare Grasseus and Fairyceus because of Psystrike. Darkrai does have a similar function that can stop Arceus from boosting up by using Dark void and resorting to Nasty Plot in attempt to overwhelm the Arceus but this is really only /effective/ in practice if the Arc is Ghost, but this is aside the point.
- +1 8 SpA Silk Scarf Arceus Judgment vs. +1 0 HP / 0 SpD Mega Mewtwo Y: 130-154 (36.8 - 43.6%) -- guaranteed 3HKO
Mewtwo has the speed and special defense to boost with CM on an Arceus that has already boosted, as shown here. If the Arceus uses Calm Mind the same turn that Mewtwo boosts, just boost alongside the Arceus until it attacks and then kill it with Psystrike. Beating CM arcs other than Ghost Arceus and the irrelevant Darkceus is a small niche but nonetheless a useful one.
Not going to mention Mmx because it seems that we are focusing on cleaning up the A ranks and Mewtwo X is irrelevant to this notion.
~
So with this evidence I've concluded about Mewtwo that it:
Pros
- Pressures hazard controlling cores exceptionally well with CM or Taunt alongside a hazards setter, with relevant set up opportunities against clerics and possesses the tools in coverage to push past different variants of these said cores (Fire blast to get past Mega Sab for example); Mega Sableye isn't as much of a liability to Mewtwo as currently thought to be by the community
- Is a versatile revenge killing threat with a niche of checking Calm Arceus formes that are not Ghost and Dark with CM of its own
- The above points plus compressing a sleep absorber and Darkrai check
Cons
- outclassed by Darkrai
- situationally worse than Skymin at checking non-espeed SD Arceus formes
- Takes up Mega
- Life Orb Mewtwo fulfills the same purposes of MMY without taking up the Mega slot, however cannot absorb sleep and check said CM Arceus formes as well
- Loses and gains some depending on the choice of Mewtwonite Y or Life Orb as an item
Mega Mewtwo Y should
remain in A and Mewtwo should
drop to A-. If a consensus is not reached that MMY is better than Mewtwo,
both should drop to A-. Seeing that both Mewtwo Y and Mewtwo is the middle ground between Darkrai (best of the three Defog stoppers) and Skymin (worst of the three), it should never be in the same rank as Darkrai, but always higher than Skymin.
E: Didn't mention Gengar at all, which may have hurt the crux of my argument. But I believe Gar is more of a Defog punisher rather than a stop to it.