As for Mewtwo EX , well you already know that Mewtwos best counter is another Mewtwo, and since Mewtwo is such a great card anyway, its advisable to run Mewtwo EX if you have any
One of the most powerful ways to do so is by making them indestructible
(Meaning that terrain-based Field Spells are applied to both sides of the field.) Each Player can control a maximum of 5 Spell/Trap cards including Pendulum cards and excluding Field Spell cards
Quicksilver Amulet is almost there on power level, and still makes decks in some Cubes, particularly those of larger sizes
This card once sold for as little as six dollars, and since then has exploded in price, climbing to ten dollars, then $15, and now currently sits at an impressive market price above $20