SASSY: +Special Defense; -Speed
Because of this nature, you've got a slow sweeper, but a moderate wall. However, Mew is all about versatility. Its true strength comes from its movepool, not its stats.
I'd put about 80 EVs into Speed (To counteract Sassy), max out Attack or Special Attack, put 88 into HP, and dump the remaining 88 into your other attacking stat for a mixed sweeper, or into Special Defense for a versatile utility fighter.
"100 Pixies" (Mew, Celebi, Jirachi, Shaymin, Manaphy, Victini) are known for their complicated EV spreads, but Mew's movepool will prove incredibly useful in battle.