Mewtwo EX/Gardevoir Ramp

The core combo of this deck is having a Mewtwo EX as the active Pokemon with two energies while having a Gardevoir on your bench. This will allow you to put two energies on Mewtwo every turn, letting you play Psydrive every turn. Once this combo activates, there aren't many decks that can tank that much reliable damage.

As a setup-dependent deck, this deck needs as much ramp as possible. For this purpose, Pokeball and Professor's Research are mandatory, and the new Mythical Slab also aids in this. Always play the ramp cards in this order: Pokeball, Mythical Slab, Professor's Research. The reason is that Pokeball will thin your deck of basic Pokemon which you don't want to waste your draw on (unless you're looking for a Ralts), then Mythical Slab will save you a draw on a psychic Pokemon while moving any other card to the bottom of your deck, and then finally you can play Professor's Research with the greatest chance of getting a card for your combo (assuming you didn't already get it from Pokeball or Mythical Slab).

The only other form of ramp in the game right now is Meowth/Sigilyph, and of the two Sigilyph has more max health, making it the obvious choice. This may be a controversial addition as by itself Sigilyph takes up a slot that can suck up an energy and thus waste a turn. In my opinion, however, any two cards you choose apart from Sigilyph will still not be the Mewtwo/Ralts/Kirlia/Gardevoir you need for the combo, and Sigilyph can at least tank some damage while allowing you to draw an additional card per turn. If you don't agree with this choice, you can always just replace them with Giovanni or Potions.