Ok a lot of people in this thread are talking about "counters". Very few ubers have counters to begin with, 2 more to add to that list isn't making that big of a difference. I mean, in generation four Skarmory was a fairly decent counter to Groudon, but with the generation shift, Groudon now uses Fire Punch a good portion of the time to deal with Ferrothorn, and Skarmory can no longer do much than pivot against Groudon.
Mewtwo has no counters. You can theorymon all you want but this statement will remain true no matter what you come up with, really. It's movepool is so big and its stats so good really nothing can counter it. These two new Pokemon have really good stats as well, but this is mitigated slightly by a vunerability to every single hazard. Ubers has always been a hazard-dominated metagame.
Now on to some *checks*:
Jirachi
Jirachi with a spread of 252 HP / 164 Def / 94 SpD and a SpD+ nature takes 37.9% - 44.6% from Black's Choice Band Outrage (assuming Jolly), and 42.1% - 49.8% from Fusion Bolt (sorry I'm not that up to date on moves, but Fusion Bolt is the 100 BP physical Electric attack, right? If not sorry, ignore this calc). If it gets Fire Punch, that does do 63.4% - 74.8% in neutral weather, but only 31.7% - 37.6% in Rain. Jirachi will take 55% - 64.9% from a Life Orb Fire Punch with neutral weather. Kyurem will certainly not enjoy paralysis, and is taking 44.5% - 52.7% from Iron Head with this spread. Now on to the calc you probably will not believe and go run yourself. Jirachi with this spread takes 41.6% - 49% from White's 100 BP Special Fire-type attack with Specs in the rain. Obviously Jirachi isn't taking this behomoth on in the sun, but in the rain its still a fairly solid check. Draco Meteor assuming this same spread and attacking Kyurem White does 43.1% - 50.7% for those that are curious (meaning Specs Blizzard won't 2HKO). Focus Blast does 49.3% - 58.2% assuming the same set for both, which has a decent potential for 2HKO, but I doubt anyone is going to be using Focus Miss when you have a 100 accuracy 100 BP Fire-type attack. Seeing as Kyogre is fairly dominant and we don't even know if Black will get Fire Punch, I'm calling Jirachi a solid non-gimmicky check for now, as this Jirachi is still fairly specially bulky as evidenced above.
Steel Arceus
Steel Arceus could probably be considered a check to Black, but 252 SpA LO Kyurem Black (120 base SpA) can do 45.9% - 54.1% to even 252/252+ Steel Arceus with Focus Blast, so thats a bit iffy. Arceus could obviously take the physical hits fine, as evidenced by its 20/20/20 boost over Jirachi, but its weakness to Fighting might be a nail in the coffin if the Kyurems decide to run it frequently. We'll see! I dont really feel like running all the calcs I did again since basically just take Jirachi's calcs and take some % off; Jirachi with 100/100/100 was surviving them so Arceus with 120/120/120 is gonna be fine. Just an item of curiosity, 252/252+ Steel Arceus takes 50% - 59% off 170 SpA 252 neutral LO 100 BP Special fire attack in neutral weather. Not enough to survive but perhaps enough to waste some PP off those 8 PP moves if needed.
Terrakion
As mentioned earlier, Terrakion is a fairly decent check, and can do 85.9% - 101.8% to even 252/0 Kyurem Black (Kyurem would need 252/140 to even have a chance of surviving with SR up lol; unlikely). With a Scarf, Terrakion will always outspeed them, but obviously can't exactly switch directly in to them, and if they have a substitute up can't do anything. Plently of other Scarfers can revenge them too, but I'm gonna leave my post at these checks because I like Terrakion :3.
As a closing comment, just Stealth Rocks + 1 layer of Toxic Spikes severely limit the staying power of the 2, chipping away 37.5% everytime they switch in and use an attack. Of course, some spikes on the field limits the usefulness of some of these checks, but they are just that: checks.
Mewtwo has no counters. You can theorymon all you want but this statement will remain true no matter what you come up with, really. It's movepool is so big and its stats so good really nothing can counter it. These two new Pokemon have really good stats as well, but this is mitigated slightly by a vunerability to every single hazard. Ubers has always been a hazard-dominated metagame.
Now on to some *checks*:
Jirachi
Jirachi with a spread of 252 HP / 164 Def / 94 SpD and a SpD+ nature takes 37.9% - 44.6% from Black's Choice Band Outrage (assuming Jolly), and 42.1% - 49.8% from Fusion Bolt (sorry I'm not that up to date on moves, but Fusion Bolt is the 100 BP physical Electric attack, right? If not sorry, ignore this calc). If it gets Fire Punch, that does do 63.4% - 74.8% in neutral weather, but only 31.7% - 37.6% in Rain. Jirachi will take 55% - 64.9% from a Life Orb Fire Punch with neutral weather. Kyurem will certainly not enjoy paralysis, and is taking 44.5% - 52.7% from Iron Head with this spread. Now on to the calc you probably will not believe and go run yourself. Jirachi with this spread takes 41.6% - 49% from White's 100 BP Special Fire-type attack with Specs in the rain. Obviously Jirachi isn't taking this behomoth on in the sun, but in the rain its still a fairly solid check. Draco Meteor assuming this same spread and attacking Kyurem White does 43.1% - 50.7% for those that are curious (meaning Specs Blizzard won't 2HKO). Focus Blast does 49.3% - 58.2% assuming the same set for both, which has a decent potential for 2HKO, but I doubt anyone is going to be using Focus Miss when you have a 100 accuracy 100 BP Fire-type attack. Seeing as Kyogre is fairly dominant and we don't even know if Black will get Fire Punch, I'm calling Jirachi a solid non-gimmicky check for now, as this Jirachi is still fairly specially bulky as evidenced above.
Steel Arceus
Steel Arceus could probably be considered a check to Black, but 252 SpA LO Kyurem Black (120 base SpA) can do 45.9% - 54.1% to even 252/252+ Steel Arceus with Focus Blast, so thats a bit iffy. Arceus could obviously take the physical hits fine, as evidenced by its 20/20/20 boost over Jirachi, but its weakness to Fighting might be a nail in the coffin if the Kyurems decide to run it frequently. We'll see! I dont really feel like running all the calcs I did again since basically just take Jirachi's calcs and take some % off; Jirachi with 100/100/100 was surviving them so Arceus with 120/120/120 is gonna be fine. Just an item of curiosity, 252/252+ Steel Arceus takes 50% - 59% off 170 SpA 252 neutral LO 100 BP Special fire attack in neutral weather. Not enough to survive but perhaps enough to waste some PP off those 8 PP moves if needed.
Terrakion
As mentioned earlier, Terrakion is a fairly decent check, and can do 85.9% - 101.8% to even 252/0 Kyurem Black (Kyurem would need 252/140 to even have a chance of surviving with SR up lol; unlikely). With a Scarf, Terrakion will always outspeed them, but obviously can't exactly switch directly in to them, and if they have a substitute up can't do anything. Plently of other Scarfers can revenge them too, but I'm gonna leave my post at these checks because I like Terrakion :3.
As a closing comment, just Stealth Rocks + 1 layer of Toxic Spikes severely limit the staying power of the 2, chipping away 37.5% everytime they switch in and use an attack. Of course, some spikes on the field limits the usefulness of some of these checks, but they are just that: checks.
