In the Pokemon TCG, Mewtwo & Mew-GX is one of the most interesting cards in the format. It doesn't actually have a non-GX move for an attack, and its GX move is fantastic. It can still attack using regular moves with its Ability Perfection.
Perfection lets Mewtwo & Mew-GX use the attacks of any Pokemon-GX or Pokemon-EX card either on the Bench or in the discard pile. It does still require the proper energy to use these attacks, however. This means that the deck uses tons of GX and EX Pokemon in order to make the most out of Mewtwo & Mew-GX's Ability.
10 Jirachi (Team Up)
Jirachi is one of the best cards that's been printed in recent years. Its Ability Stellar Wish is what pushes it into the top echelon of cards. In Pokemon, Trainer cards are the best kind of cards to see, as they all have incredibly powerful effects that can get your plays going. Jirachi lets you look at the top 5 cards of your deck and add any kind of Trainer card from there to your hand. Despite its simplicity, when getting to your playmaker cards early is so important, Jirachi's Ability is so good for getting to them.
9 Professor's Research
Professor's Research is already a fantastic Supporter card, but in Mewtwo & Mew-GX decks, in particular, it's especially amazing. Since Mewtwo & Mew-GX can use the attacks of GX and EX Pokemon in the discard pile, its effect to discard isn't even a downside.
Not only can Professor's Research mulligan your hand, but still open up new plays thanks to it powering up Mewtwo & Mew-GX's Ability. In addition to that, it can even combo off with other cards in the deck, giving you a ton of advantage with just one use.
8 Quick Ball
Quick Ball is one of the best Trainer cards in the game. At the cost of discarding a card, you can search out any Basic Pokemon from the deck and add it to your hand. Mewtwo & Mew-GX, as well as all Tag Team Pokemon, are considered Basic, meaning it lets you get to them incredibly quickly.
Since you get to choose what card you discard, you can get rid of a GX or EX Pokemon in your hand, so there's no risk of it getting knocked out on the Bench, and Mewtwo and Mew-GX can take advantage of its attacks.
7 Giant Hearth
Even though Mewtwo & Mew-GX are Psychic-types, there isn't actually many great EX or GX Pokemon of that type. As such, the deck runs other types along with them. The main type played with it is Fire. Fire-types have a lot of fantastic GX Pokemon, as well as some of the best support cards in the game.
Giant Hearth is one of these cards. You can discard a card, then add 2 Fire-type Energy cards from your deck to your hand. This sets up the discard pile for Mewtwo & Mew-GX, as well as giving you Energy advantage with the ones it adds.
6 Cherish Ball
Cherish Ball is incredibly simple. All it does is let you search out any Pokemon-GX, reveal it, and add it into your hand. Despite its simplicity, Cherish Ball is fantastic in a deck that relies on a ton of GX Pokemon.
There's no downside to using it either. Unlike Quick Ball, Cherish Ball doesn't require any discards so you break even in card advantage by using it. Cherish Ball lets you get any GX Pokemon that you need for a specific situation, allowing you to use any attack right after with Mewtwo & Mew-GX.
5 Incineroar-GX
Incineroar-GX is a Dark-type Pokemon that luckily uses Colorless Energy for all its attacks. Even though it's an Evolution, Mewtwo & Mew-GX copying its attacks make it worth playing since it's easy to get it in the discard pile. Its normal attack is situational, but 130 is solid damage and when the effect to discard a Special Energy from the opponent comes up it's great.
Incineroar-GX's GX attack is what puts it over the edge. For each damage counter on it, Incineroar-GX does 50 damage for each one (plus a base power of 10). Since GX Pokemon have so much health, this will almost guarantee a knock out whenever it's used.
4 Horror Psychic Energy
Horror Psychic Energy is the Special Energy for the Psychic-type, and one that benefits it greatly. It provides Psychic Energy, and anytime the Pokemon is attached to is damaged, it puts 2 damage counters on the Attacking Pokemon (so long as it was the Active Pokemon)
This may seem incredibly minor at first glance, but Pokemon is a game of inches. Galarian Zigzagoon is one of the most played cards in the game for the fact it lets you put 1 damage counter on a Pokemon. So, Horror Psychic Energy is even better, as it provides Energy to use attacks without putting an easy to knock out target on the Bench.
3 Espeon & Deoxys-GX
Espeon & Deoxys-GX isn't a fantastic Pokemon at first glance, since its regular attack is only beneficial if you have a lot of Psychic Pokemon on the Bench. Since you don't actually run many Psychic-type Pokemon in the Mewtwo & Mew-GX deck, you won't be using it much.
What you will be using is its GX attack Cross Division GX. It allows you to put 10 damage counters on any of your opponent's Pokemon in any way you want. If it has 3 extra Energy Cards attached to it for the attack, then it lets you choose 20 damage counters instead.
2 Reshiram & Charizard-GX
Reshiram & Charizard-GX is a card so good a deck based around it is already fantastic. Mewtwo & Mew-GX can make amazing use of it, especially because it can easily play around the downside of not being able to use Flare Strike the next turn.
Reshiram & Charizard-GX give Mewtwo & Mew-GX a slew of powerful moves to choose from, and can easily deal a minimum of 230 damage. Its GX move, Double Blaze GX can also easily knock out opposing Pokemon without a second thought. Reshiram & Charizard-GX is a great "Plan B" for the deck, all while supporting the main strategy.
1 Welder
As a rule of thumb, if a deck plays any Fire-Type Pokemon, you should be playing as many copies as Welder as possible. Welder is one of the best cards ever printed and the best support cards for Fire-types. It lets you attach up to 2 Fire Energy cards to a Pokemon, and draw 3 cards afterward.
Not only does this give you a massive boost of Energy presence on the field, but you get back all the cards that left your hand with the three cards drawn. Welder gives you insane levels of advantage, and it's amazing that Welder ever got past R&D and into the game.