Reserving
Offensive & Defensive Mew.
Edit: The Offensive is finished.
Edit 2: The Defensive is finished.
Offensive
Mew
Type: Psychic
Base Stats: 100 / 100 / 100 / 100 / 100 / 100
Ability: Synchronize
Sparring a definite fine 100 All-Round base stats, and a movepool behind comprehension, Mew surely is a force to be reckoned with. With these stats and movepool, it allows it to confuse opponents making them think if it is Offensive, or Defensive. It also has two great boosting moves being Nasty Plot, and Swords Dance. With it's near infinite movepool, its Nasty Plot set have many moves to choose from. Psyshock tends to be a staple to hit special walls. However, you may use Psychic if your team will not have trouble defeating special walls. Fire Blast tends to be the best option to destroy Steel Types that would otherwise eat the Psychic moves. Giga Drain can be the final option to have extra recovery and hit bulky Water Types at the same time, however, Dark Pulse and Aura Sphere can be used over it to hit Pokemon that would otherwise resist the set, such as Victini, and Umbreon/Mega Houndoom respectively. Mew can also run a viable Swords Dance set which can be as destructive as it's Nasty Plot set! With Sucker Punch and 100 Base Speed, it's one of the best priority users of UU! Zen Headbutt is Mew's main STAB attack, which can OHKO many of the tier's threats with +2. Finally, the last slot can be Drain Punch, which helps it deal with Dark Types that would normally counter this set completely! With all of this in mind, Mew can always be your winning condition in X/Y UU.
Defensive
Mew
Type: Psychic
Base Stats: 100 / 100 / 100 / 100 / 100 / 100
Ability: Synchronize
Falling in grace from D/P Ubers, Mew finds itself in UU, but don't think that it makes it any less viable. All-Round 100 Base Stats, and a near endless movepool allows Mew to support itself, and it's team, better then almost any Pokemon in UU. With decent 100 Base Stats, Mew can be both a Physical, or Special Tank, either work extremely well and will depend on what your team will enjoy more. To make Mew even more frightening - few Pokemon can ever safely cripple it depending on it's type due to it's ability Synchronize! Mew may not sport the best bulk in 100/100/100, but Mew can stop many sweepers as well! With it's absurd movepool offering it Will-O-Wisp, and Thunder Wave to cripple them, not many sweepers can come in safely! Now Mew can do all this, it has much more! It has Roost/Soft-Boiled to increase it's longevity, it has Taunt to stop slower walls/tanks. It also learns Roar if you want to phaze things, which many tanks can wish for. In general, Mew should never be overlooked as with it's movepool and decent stats, it can make many teams extremely dangerous, every team should be ready for it.