Mew is so versatile, it makes Abomasnow's movepool look like a Beldum.
Set name: Rock Polish Sweeper
Mew @ Life Orb
EVs: 228 Def / 252 SAtk / 28 Spe
IVs: 29 HP
-Thunderbolt / Roost / Shadow Ball / Baton Pass
Scarfed Pokemon typically revenge kill Mew with ease. However, this set fixes that by doubling Mew's speed to outrun the entire unboosted UU. Rock Polish is obviously how the set got its name, it allows Mew to get the speed jump on many slower scarfed Pokemon even without max speed investment. Psychic is good STAB and with Life Orb OHKOs Heracross and Mienshao, two of UU's biggest threats, while hitting hard the majority of the tier. Aura Sphere provides essential coverage against Dark-types and makes this set incredible: lure in Dark-types as you boost while KO-ing them, and it really surprises those scarfed Krookodiles. The last move is a matter of choice; Thunderbolt guarantees the OHKO on Honchkrow, and allows you to hit Slowbro pretty hard. Roost is excellent in tandem with Life Orb, keeping Mew healthy for extended periods. However, do note that Sableye, Spiritomb and Victini will utterly wall this set so your team needs to be able to kill them if you run Roost. Next comes Shadow Ball, which allows you to hit Victini and other Mews harder than any other move, and also hits Slowbro hard. Do note that Thunderbolt has a higher base power, however, and unless your team has huge problems with Victini, Thunderbolt is almost always superior. finally comes Baton Pass, which is very useful when coming across something that walls you (middle finger to Shedinja and Meloetta) and possibly passes the boosts to something else and set up a sweep.
Life Orb is there because it turns many 2HKOs into OHKOs. Technically you could run Sub+Leftovers but the power drop is notable, although you do get insurance vs status and dark-types. 252 SAtk EVs to hit as hard as possible, while 28 Spe EVs outspeed non-scarf Chandelure before a boost, Modest because the extra speed boost is hardly needed. 29 HP IVs obtain a Life Orb number. The rest of the EVs are dumped into Defence to allow more switching power into choice-locked fighting type attacks, opening up more opportunities for set-up.
Stealth Rocks should be up at the other end of the field. This helps tremendously in KOing stuff, most especially Chandelure who could otherwise OHKO Mew in return. A layer or two of spikes are also recommended because this set appreciates residual damage a lot. This set has trouble with special walls such as Snorlax and Umbreon, thus you can run Toxic in the last slot (but never, ever use Toxic on Umbreon), or Superpower with a different EV spread. Having teammates to take them down is also amazing, and Spikes support really helps in this cause.
Dark-type Pursuit users are also great teammates to take out the Psychic-types that wall Mew.
I'll post the calcs when I've more time