Mew EX
Mew EX is... a little odd. It's genuinely great, but its real utility depends heavily on the kind of deck your opponent is running. A well timed Mew EX can hard-counter decks like Charizard EX with relative ease, turning the fire lizard's tremendous offensive power against it without any repercussions, and it can also be used to copy useful effect-spreading attacks when facing down something like Wigglytuff EX. However, there will be times when it naturally doesn't pan out so well, such as against Pikachu EX due to the fact that you're pretty unlikely to have a bench full of Electric Type cards to boost its attack when running Mew EX.
All-in-all, Mew EX can definitely work as the centerpiece for a deck, but it's generally best utilized as an insurance policy lurking on your bench, ready to copy something dangerous and turn the tide of battle in an instant. And in this regard, Mew's need for colorless energy means it can potentially work in virtually any deck as a secondary win-condition.
Mew EX + Mewtwo EXGo to Deck
While honestly a bit more of a Mewtwo EX deck than a Mew EX deck, Mew EX gives a bit of a unique flavor. It can come in early-game and wall for its team and then drop from play with Budding Expeditioner to make way for Mewtwo EX to sweep, and/or it can sit on the bench and amass energy, ready to come in and potentially counter any big-name cards that may otherwise be threatening a sweep. The best part is the fact that having even one Gardevoir on the bench makes the heavy energy investment needed far more manageable.