It is ridiculous what this thing can survive. It can take pretty much every Choice Scarf attack at full health (except maybe Sazandora's Choice Scarf Dark Pulse, although, who the heck uses Dark Pulse on a Choice Scarf Sazandora?) and then murder you. Even with MIN/MIN, this thing survives pretty much everything and with Nasty Plot, can proceed to sweep you, which I would love to see to show people how uber this thing is. Same with Manaphy but this is about Mew. Note that these calculations could also be made for Manaphy (except the Scarf Dark Pulse which is neutral on Manaphy).
Choice Scarf Garchomp Outrage: 57.31-67.84% Ko with Icebeam
Choice Scarf Ononokonsu Outrage: 62.57-73.98% Ko with Icebeam
Choice Band Adamant Onononkonsue Outrage 103.51-121.93% (but it will survive behind Reflect), faster, Ko with Icebeam
Choice Scarf Tyranitar Pursuit if you stay in: 39.18%-47.37% (because leading off with Crunch is dangerous, especially now with Justice Heart) OHKO with +2 Life Orb Aura Sphere
Damage tp Erufuun unboosted Flamethrower/Icebeam does: Always 2 hit ko unless they don't invest in hp and special defense at all in which case, Life Orb unboosted OHKOs.
Choice Scarf Sazandora Draco Meteor: 64.91-76.61%, Ko with +2 Aura Sphere/Icebeam with Life Orb, unboosted and Life Orb, 75.69%-89.23%
Choice Scarf Sazandora Dark Pulse: 74.27-88.30%
Choice Band Technician Scizor Bullet Punch: 47.37%-56.14%, only 20% chance for 2 hit ko if running Leftovers. Kill with Flamethrower, or heavily dent with +2 Life Orb Aura Sphere for 74.34-87.76%.
And behind Screens and with Max hp and 72 evs in defense invested:
Adamant Choice Band Tyranitar Crunch: 50.50-59.90%
Swords Dance +2 Life Orb Adamant Ononokonsu Outrage: 70.74-83.17%
Choice Scarf Tyranitar Crunch (who by the way does not OHKO min/min Mew with Crunch even with Sandstorm unless it has Sandstorm damage):31.19-37.13%
Mew also is an amazing Screener. It is even better than Cresselia who you'd at first think would be better because of higher hp and defenses. However, Mew has speed, and something Cresselia would kill for: a good recovery move that actually works in Sandstorm and weather. In any weather but Sun, Moonlight is bad only restoring 25% health. Roost/Softboiled will always heal 50%. Also, Mew can also scout in and out with U-Turn, instead of switching which also dents Tyranitar and Erufuun a little instead of just switching. It is a reuseable screener, and Mew is very bulky.
MEW IS GOOD AT BULK AND OFFENSE. Few things can outrun it that can ko it and a lot of them are Choice Scarfers. Choice Scarfers can't ko a min/min and Mew is bulkier than Swampert. Plus, it outruns most dragons and can kill them with an easy Icebeam as well as having a move for anything else you want to cover with Flamethrower, Thunderbolt, Psycho Shock (which isn't entirely necessary but fun if Blissey shows up again), Psychic, Dragon Pulse, or physical options like Dragon Tail, Dragon Claw, Outrage, Earthquake, Drain Punch, Zen Heabutt, Wild Bolt, Waterfall, Ice Punch, Thunder Punch, Fire Punch, and on and on.
I don't see the point of Block sets when you can just lure Tyranitar in, then U-Turn out to something that can scare it out and boost. If you're block-U-Turning, why not just U-Turn and save a move slot? Also, Nasty Plot Life Orb Aura Sphere will ALWAYS kill it, no matter what set it's running. Scizor is heavily dented/koed because most people aren't going to use Bug Bite. Ditto doesn't work so well because Mew has no moves to hit super-effective on itself (with sets like Aura Sphere/Icebeam, Flamethrower/Psycho Shock, etc) and with +2 Icebeam/Flamethrower does 50.58-59.65%. Then you know what it will choose to run because it's locked in and switch in your own Pursuiter (Tyranitar would be fun) and watch Ditto die. Erufuun can be annoying but it risks death and you can do the Magic Coat thing (when did Mew get Magic Coat anyways?). Mew is amazing at attacking and defense. This thing is way amazing. It is Uber.
I'm still praying for it to get Mischievious Heart next game like it should have gotten that would overwhelmingly make it Uber and actually good there. It is STILL GOOD there. People seriously underestimate Mew because it was bad in Ubers...only because Ubers smack with 150 base attacking stats, double stabbed rain surfs, Swords Extremespeeds and so on and so forth. It is hard for Mew to compete with that there. But in OU...oh god. Why aren't people using this when they can? It is so freaking cute and scary! But cute!