Mew

Type Tier
Psychic Uber
Statistics
Stat
HP
100
403
Atk
100
298
Def
100
298
SpA
100
298
SpD
100
298
Spe
100
298
Name Item

Physical Sweeper Mew

Leftovers
Moveset
~ Swords Dance
~ Submission
~ Softboiled
~ Shadow Ball / Rock Slide

OK, this is simple enough. Swords Dance up and hit everything with the attack that will do the most damage. Submission will work on most things. Shadow Ball is for the Psychics that are prominent in the uber metagame, and Rock Slide will take apart the Flying legendaries.

Other Options

If Shadow Ball or Rock Slide doesn't appeal to you, you've got just about every special attack under the sun to use to counter whatever you want to. You can also use Swords Dance with Earthquake or Explosion. There's also a myriad of other good moves you can use—some examples are Curse, Counter, and Thunder Wave. It's also possible to make any other combination of these, but in general the physical sweeper set is the most effective overall.

Opinion

Mew is probably the best Uber due to Swords Dance and the movepool that allows it to do anything it wants. It's best used as a physical sweeper since most physical walls and the other ubers/legendaries have a physical weakness that Mew can exploit.

Counters

There really isn't a sure-fire Mew counter, and that's why he's uber. Your best bet is probably Heracross, since he resists Submission and most Mews don't have Psychic or Fire Blast. If Mew doesn't have Psychic or Earthquake, then Muk stands to be a good counter too.